rf)t n (7hit rOu @n. r~i'nlirf)lin 1: Jll u

rf) n '~JIiu r()n (dng. '~i'ti(J)hn 2: r7nlt ton cc c trng tuig on th li

#/20

#/20

rf)t n Ckit rOu @n. r~i'nlirf)lin 1: Jll u

(St): 'T Quang 'TiU Jlp

3UaQrnh

#/20

N MON HC THUY VN CONG TRNHTnh ton thu vn thit k h cha

I. M U

Xut pht t yu cu thc tin i vi cc vn v ti tiu, cung cp nc cng trnh h cha nc ni Cc d kin xy dng trn sng Cng nm trn a bn tnh Thi Nguyn, pha ng Bc t quc.

Vi nhim v thit k h cha, chng ta tin hnh o c v quan trc trn lu vc sng Cng v thu c cc ti liu nh sau:

Din tch lu vc n tuyn cng trnh l F = 535 (km2).Chiu di sng chnh l Ls = 56.0 (km).Lng ma ngy thit k l Hnp = 475.1 (mm).Tc dng chy l ln nht o c ti mt ct ca ra ca lu vc Vmax = 2.85 (m/s).Tin hnh quan trc trong 16 nm (t 1961 -- 1976) c lu lng bnh qun thng cho nh bng 1.Qu trnh l o c c trn sng Cng ti tuyn d nh xy dng cng trnh cho nh bng 2.Ti liu qu trnh nc dng cho nh bng 3.Ti liu tn tht: Phn phi chnh lch bc hi cho nh bng 4.Quan h c trng a hnh lng h cho nh bng 5.

Cng vic thit k i hi cc yu cu sau:

Tnh ton cc c trng thu vn thit k h cha.Tnh ton cc thnh phn dung tch h cha.

Cc ch s'k thut ly theo yu cu thit k:

Tiu chun thm k = 1%.p trn x l gm 3 khoang, chiu rng mi khoang b = 8.0 (m), ngng trn cao trnh Ztr = 41.2 (m). Mc nc trc khi l v mc nc dng bnh thng

( Zt, = Hbt).

H cha thit k vi cao trnh mc nc cht Hc = 34.0 (m), tng ng vi Vc = 7.5 X 106 ( m3).

Bng 1: Lu tng bnh qun thng (m3/s) ca /u vc sng cng.

Xrhng

Nm\

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

BQ

nm

1961

2.50

4.12

7.14

14.00

6.73

34.90

14.10

40.60

55.20

21.70

12.80

8.48

18.523

1962

4.19

2.59

2.79

4.36

4.99

23.10

20.00

22.80

20.90

9.56

3.10

1.41

9.9825

1963

0.87

1.50

1.80

1.48

10.70

9.77

51.80

35.60

21.90

9.89

18.20

6.62

14.178

1964

3.04

3.66

3.93

9.77

6.12

19.70

32.50

25.60

49.10

40.00

11.40

7.36

17.682

1965

3.11

3.25

3.44

20.20

18.10

41.40

24.90

11.90

13.70

15.10

6.88

4.44

13.868

1966

4.10

2.94

1.66

5.94

4.18

37.10

23.20

23.60

12.60

22.80

6.89

3.67

12.39

1967

1.71

4.02

4.03

8.31

9.04

14.40

15.00

18.90

35.80

6.33

4.21

2.57

10.36

1968

2.02

4.19

5.09

8.35

18.70

19.70

15.90

83.90

26.20

27.10

10.60

3.18

18.744

1969

3.22

2.31

4.20

9.42

13.20

13.60

29.40

44.00

25.00

10.60

16.00

3.08

14.503

1970

2.61

3.10

3.16

8.48

18.20

30.10

23.80

26.10

21.10

12.20

4.35

2.61

12.984

1971

2.06

2.64

3.88

3.11

29.70

15.60

71.30

91.50

20.20

24.00

5.77

2.75

22.709

1972

2.07

2.33

2.03

3.74

19.40

9.30

5.94

82.30

35.30

18.30

9.33

4.27

16.193

1973

3.46

3.41

3.04

12.40

9.02

24.00

49.60

53.80

84.50

17.70

5.76

2.74

22.453

1974

3.43

2.76

2.56

7.11

7.04

23.00

21.50

16.30

12.80

17.50

5.28

3.06

10.195

1975

5.97

2.74

5.26

13.90

48.20

53.40

10.50

22.40

38.60

9.63

4.52

2.66

18.148

1976

1.63

4.83

2.08

8.58

13.00

6.24

2.96

27.20

27.00

20.30

13.30

2.94

10.838

Bng 2: Qu trnh l thc o trn sng cng (t 7h ngy 23 n 13h ngy 28/VII/1971).

Thi on

1

2

3

4

5

6

7

8

9

10

11

12

Q (m3/s)

16

131

266

529

720

437

240

205

170

137

110

83.8

Thi on

13

14

15

16

17

18

19

20

21

22

Ghi ch

Q (m3/s)

69

59

52

44

42

38

34

33

32

33

At = 6 gi

Bng 3: Qu trnh nc dng (q ~ t).

Thng

I

II

III

IV

V

VI

VII

VII

IX

X

XI

XII

BQ Nm

q(m3/s)

18

20

13

11

7.6

6.6

4.6

5

5.7

6.5

8

12.5

9.92

Bng 4: Phn phi chnh lch bc hi (AZ ~ t).

Thng

I

II

III

IV

V

VI

VII

VII

IX

X

XI

XII

AZ Nm

AZ (mm)

13.1

10.8

10.7

11.1

21.5

19.2

19.0

17.5

17.5

19.1

13.3

12.2

185.0

(St): 'T Quang 'TU Jlp

3Uioa @ng r~t'tili

Bng 5: Cc quan h c trng lng h (Z ~ F) v (Z /V V).

Z(m)

34.0

35.0

36.0

37.0

38.0

39.0

40.0

41.0

41.2

42.0

F (km2)

4.2

5.0

6.5

8.0

9.6

11.6

13.5

15.5

15.8

17.2

V (10* m3)

7.5

10.5

16.8

25.0

33.5

43.5

55.0

70.0

73.2

86.0

Z(m)

43.0

44.0

45.0

46.0

46.2

46.5

47.0

48.0

49.0

50.0

F (km2)

19.0

20.6

22.5

24.6

25.0

25.7

26.8

29.6

32.3

35.0

V (10* m3)

103.8

125.5

147.8

172.0

175.5

183.8

198.0

224.9

250.5

280.0

II. TNH TON CC C TRNG THU VN THIT K H CHA

Phn ma dng chv

T ti liu dng chy thng quan trc c trn lu vc trong 16 nm (t 1961 --1976), ta xc nh c ma dng chy cho lu vc nh sau:

\Thng Nm \

I

II

III

IV

V

VI

VII

VIII

IX

X

XI

XII

BQ

nm

1961

2.50

4.12

7.14

14.00

6.73

34.90

14.10

40.60

55.20

21.70

12.80

8.48

18.523

1962

4.19

2.59

2.79

4.36

4.99

23.10

20.00

22.80

20.90

9.56

3.10

1.41

9.9825

1963

0.87

1.50

1.80

1.48

10.70

9.77

51.80

35.60

21.90

9.89

18.20

6.62

14.178

1964

3.04

3.66

3.93

9.77

6.12

19.70

32.50

25.60

49.10

40.00

11.40

7.36

17.682

1965

3.11

3.25

3.44

20.20

18.10

41.40

24.90

11.90

13.70

15.10

6.88

4.44

13.868

1966

4.10

2.94

1.66

5.94

4.18

37.10

23.20

23.60

12.60

22.80

6.89

3.67

12.39

1967

1.71

4.02

4.03

8.31

9.04

14.40

15.00

18.90

35.80

6.33

4.21

2.57

10.36

1968

2.02

4.19

5.09

8.35

18.70

19.70

15.90

83.90

26.20

27.10

10.60

3.18

18.744

1969

3.22

2.31

4.20

9.42

13.20

13.60

29.40

44.00

25.00

10.60

16.00

3.08

14.503

1970

2.61

3.10

3.16

8.48

18.20

30.10

23.80

26.10

21.10

12.20

4.35

2.61

12.984

1971

2.06

2.64

3.88

3.11

29.70

15.60

71.30

91.50

20.20

24.00

5.77

2.75

22.709

1972

2.07

2.33

2.03

3.74

19.40

9.30

5.94

82.30

35.30

18.30

9.33

4.27

16.193

1973

3.46

3.41

3.04

12.40

9.02

24.00

49.60

53.80

84.50

17.70

5.76

2.74

22.453

1974

3.43

2.76

2.56

7.11

7.04

23.00

21.50

16.30

12.80

17.50

5.28

3.06

10.195

1975

5.97

2.74

5.26

13.90

48.20

53.40

10.50

22.40

38.60

9.63

4.52

2.66

18.148

1976

1.63

4.83

2.08

8.58

13.00

6.24

2.96

27.20

27.00

20.30

13.30

2.94

10.838

K.tra

0.06

0.375

0.69

0.69

0.94

0.81

0.563

0.13

15.234

Chn ch tiu so snh l lu lng bnh qun nm bnh qun trong nhiu nm:

o=xe, = 15.234 (m3/5)

w i = 1

(St): 'T Quang 'TiU Jlp

3Cla c/ng r~t'tilirf) n jhug (Z)n (tng 'r nilrf)lin 2: Jinh ton ee e trng, thug on thit h'

rf) n jhug (Z)n (tng 'r nilrf)lin 2: Jinh ton ee e trng, thug on thit h'

#/20

#/20

r n 7tuu/ r()n @4tg Tenhr()lttn 2: QiiA ttn ee te trng, thiu/ on th k'

#/20

iu kin chn mt thng l thng ma IL7 hay ma kit:

0 Nu P(Qi> Qo) > 50% th thng l thng ma l.

0 Cc thng cn li l cc thng ma kit.

Kt qu cho ta:

=> T thng VI -h thng X: Ma l.

=> T thng XI thng V: Ma kit.

Ma

Ma l

Ma kit

Nm

VI

VII

Vin

IX

X

XI

XII

I

n

III

IV

V

61-62

34.9

14.1

40.6

55.2

21.7

12.80

8.48

4.19

2.59

2.79

4.36

4.99

62-63

23.1

20

22.8

20.9

9.56

3.10

1.41

0.87

1.50

1.80

1.48

10.70

63-64

9.77

51.8

35.6

21.9

9.89

18.2

6.62

3.04

3.66

3.93

9.77

6.12

64-65

19.7

32.5

25.6

49.1

40

11.4

7.36

3.11

3.25

3.44

20.2

18.1

65-66

41.4

24.9

11.9

13.7

15.1

6.88

4.44

4.10

2.94

1.66

5.94

4.18

66-67

37.1

23.2

23.6

12.6

22.8

6.89

3.67

1.71

4.02

4.03

8.31

9.04

67-68

14.4

15

18.9

35.8

6.33

4.21

2.57

2.02

4.19

5.09

8.35

18.7

68-69

19.7

15.9

83.9

26.2

27.1

10.6

3.18

3.22

2.31

4.20

9.42

13.2

69-70

13.6

29.4

44

25

10.6

16

3.08

2.61

3.10

3.16

8.48

18.2

70-71

30.1

23.8

26.1

21.1

12.2

4.35

2.61

2.06

2.64

3.88

3.11

29.7

71-72

15.6

71.3

91.5

20.2

24

5.77

2.75

2.07

2.33

2.03

3.74

19.4

72-73

9.3

5.94

82.3

35.3

18.3

9.33

4.27

3.46

3.41

3.04

12.4

9.02

73-74

24

49.6

53.8

84.5

17.7

5.76

2.74

3.43

2.76

2.56

7.11

7.04

74-75

23

21.5

16.3

12.8

17.5

5.28

3.06

5.97

2.74

5.26

13.9

48.2

75-76

53.4

10.5

22.4

38.6

9.63

4.52

2.66

1.63

4.83

2.08

8.58

13.00

76-77

6.24

2.96

27.2

27

20.3

13.30

2.94

Tnh lng dng chv ca tng ma cho tng nm cho tt c cc nm o ac c dng chy hng thng

Ql ~ (Qvi "* Qvn Qviii Qix Qx) /5

Qn - (Qi *" Qn Qui + Qiv Qv Qxi Qxii ) 17

WLi =5x0^x86400 (ot3)

WKl =7x^x86400 (ff3)

wl = WLi + WKi

Ta c bng sau: (St): 'T Quang UiU Jlp 42TI:f 3Uioa (^(ti TittltMa

Ma l

Ma kit

C nm

Qi

WjX 106

Qi

WjX 106

Qi

WjX 106

61-62

33.30

431.57

5J4

104.20

17.23

535.77

62-63

19.27

249.77

2.98

54.07

9.77

303.83

63-64

25.79

334.26

7.33

133.07

15.03

467.34

64-65

33.38

432.60

9.55

173.30

19.48

605.91

65-66

21.40

277.34

4.31

78.12

11.43

355.47

66-67

23.86

309.23

5.38

97.64

13.08

406.87

67-68

18.09

234.39

6.45

116.98

11.30

351.37

68-69

34.56

447.90

6.59

119.57

18.24

567.47

69-70

24.52

317.78

7.80

141.60

14.77

459.38

70-71

22.66

293.67

6.91

125.32

13.47

419.00

71-72

44.52

576.98

5.44

98.73

21.72

675.71

72-73

30.23

391.75

6.42

116.46

16.34

508.21

73-74

45.92

595.12

4.49

81.39

21.75

676.51

74-75

18.22

236.13

12.06

218.79

14.63

454.92

75-76

26.91

348.70

5.33

96.68

14.32

445.38

76-77

16.74

216.95

0.00

0.00

0.00

0.00

Dng cc it s liu tnh ton c trn xv dng cc ng tn sut tng ng theo cc bc sau v:

Xy dng ng tn sut kinh nghim:

Sp xp cc gi tr WLj, W| ti = 8.0 (h)

H s hnh dng l f c ly theo bn phn khu h s hnh dng l

(f = 0.8)

H s trit gim s c tnh bi cng thc:

5 =!

+ CXfaoho

(V khng c ti liu v ao h, m ly => chn 5 = 1)

Qng = Qo = 15.234 (m3/s)Tra bng quan h ma ro dng chy ti cc phn khu ta thy sng Cng thuc phn khu VI nn OL = 0.72; Ho = 24.Htp tnh theo cng thc Hp = \|/(T) X Hnp

Vi T = ti = 8.0 (h) => \|/(T) = 0.79 => Hp = 0.79x475.1 = 375.33 (mm)

=> =mi% =0.278x-72x(3785-33~24)x0.8xlx535 + 15.23

=> Qmaxi% = 3777.5(m3/s)

2. Tnh tng lng l thit k' Wm^n

Tnh Wmaxp theo cng thc: wmaxp = 103XHTpXFXa

Trong :

(X l h s dng chy tng lng, c th xc nh c thng qua bng tra h s dng chy.(Vi Hnp 475.1 > 200, F > 100 km2 ta xc nh c (X = 0.9).

HTp = 1/(T) X Hnp = 375.33 (mm).

=> Wmaxp = 103x 375.33 X 535 X 0.9 = 180721395 (m3).

3. ng au trnh nh thit k

Do lu vc c sn ti liu v l nn y ta xy dng ng qu trnh nh l theo phng php da vo qu trnh l i biu.

y ta chn qu trnh l i biu l qu trnh l thc o trn sng Cng (t 7h ngy 23 n 13h ngy 28/VII/1971).

Q (m3/s)

Da vo ng qu trnh l in hnh trn, ta xc nh c: Qmaxh = 720 (m /s)

VVmaxh = 74838600 (m3)

Theo tnh ton trn, ta c cc gi tr l thit k:

Qma*p= 3777.5 (m3/s)

Wmaxp= 180721395 (m3)

=> Ta xc nh c cc h s thu phng:

3777 5

K0 == = 5.24653

L,* 720

(St): 3 Quang, 3ug Jlp 42(:f DChoa (Qng rnhrO n Chin ()n (m r7i'tilirf)hn 4: Cjnlt ttn ce thnh phn dun, teh h ehtL

rf) n C/hin ()n (m r7i'tilirf)hn 4: Cjnlt ttn ce thnh phn dun, teh h ehtL

#/20

(St): 'T Quang 'TiU Jlp 42(/..,f

3Ua CJn Ti t th

#/20

rf)t n Ckit (10).

Ct (12) l lng nc thiu c tnh theo cng thc:

AV = (10) - (9) nu (10) > (9).

Ct (13) l dung tch cha nc ca h cha c tnh bng cch tch lu ct (11) nhng ch khng c vt qu dung tch h.Ct (14) l dung tch x nc ca h cha (l dung tch tha khi h cha tch nc).

Vy t bng tnh, ta c th xc nh c gi tr ca dung tch hiu dng ca h cha l:

(V y l h cha iu tit mt ln nn dung tch hiu dng ca h cha ng bng tng lng nc thiu trong nm).

Vh =v~ =153.38x106(w3)

2. Tnh ton iu tit l

Nhim v ca bi ton ny thc cht l tnh ton xc nh dung tch phng l cn thit, tc l tnh ton dung tch siu cao, mc nc siu cao ca h cha nhm m bo nhu cu phng l ca cng trnh h cha. VI kch thc v qui m cng trnh xc nh c bn nh sau:

Chiu rng ca p b = 24 (m).Cao trnh ca ngng trn l Ztr = 41.2 (m).Cao trnh mc nc cht Hc = 34.0 (m).Qu trnh l thit k tnh ton c phn trc

Thi on

1

2

3

4

5

6

7

8

9

10

11

12

Q (m3/s)

83.9

687.3

1395.6

2775.4

3777.5

2292.7

1259.2

1075.5

891.9

718.8

577.1

439.7

Thi on

13

14

15

16

17

18

19

20

21

22

Ghi ch

Q (m3/s)

360.4

308.5

273.9

229.8

219.8

199.4

179.4

174.7

170.0

174.7

At = 2.8 gi

T , ta c th tnh ton iu tit l theo phng php th dn nh sau: a. xv dng biu quan h z ~ V theo s liu xc inh c bng 5

z~ V

Xv dng biu b tr

xy dng c biu b tr trc ht ta phi lp bng tnh cc gi tr b tr.

z

(m)

h

(m)

q

(m3/s)

Vk

(106m3)

V

(106m3)

fl(q)

f2(q)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

41.2

0.0

0.00

73.2

0.0

0.00

0.00

42.0

0.8

37.27

86.0

12.8

1251.20

1288.48

43.0

1.8

125.80

103.8

30.6

2972.82

3098.61

44.0

2.8

244.06

125.5

52.3

5066.46

5310.52

45.0

3.8

385.86

147.8

74.6

7207.86

7593.72

46.0

4.8

547.80

172.0

98.8

9527.69

10075.49

46.2

5.0

582.39

175.5

102.3

9857.62

10440.00

46.5

5.3

635.58

183.8

110.6

10654.43

11290.01

47.0

5.8

727.61

198.0

124.8

12017.15

12744.76

48.0

6.8

923.68

224.9

151.7

14587.76

15511.44

49.0

7.8

1134.75

250.5

177.3

17021.91

18156.66

50.0

8.8

1359.82

280.0

206.8

19835.96

21195.78

Trong :

Ct (1) l gi thit cc mc nc trong h cha z (m).Ct (2) l chnh lch gia cao trnh mc nc thc t vi cao trnh ngng trn h = z - Ztr (m).Ct (3) l lu lng x q (m3/s) c tnh bng cng thc:

q = mxbXyxg xh/2

Ct (4) l dung tch ca kho nc Vk (m3) ng vi mc nc c trong kho v c xc nh trc tip t biu quan h z ~ V.Ct (5) l dung tch trn ra khi h cha V (m3) ng vi tng gi tr mc nc trong kho v c xc nh theo cng thc V = Vk - vtr.Ct (6) v ct (7) l gi tr cc hm b tr fl(q) v f2(q) c xc nh theo cng thc:rO n r~hin ()n (m r7i'tilirf)hn 4: (inh ttn ce thmh phn dung teh h ehtL

r n r7tnu'i r()n (^tt r~Ji'nli(J)hn 4: ^nh ton cc thnh phn lun. tch h cha

(St): 'T Quang, 'Tug Jlp 42C,f

3Ua (ng ^i t ti

#/20

#/20

(St): 'T Quang, 'Tug Jlp 42(/,:f

3CJha (/ng Tvhih

rO n r~hin ()n (m r7i'tilirf)hn 4: (inh ttn ce thmh phn dung. teh h ehtL

(St): 'T QfjUin. 'Ti Jlp 42(:f

3Ua CJn ^itth

#/20

v_

At

f2(q) = +

A t 2

Da vo cc gi tr tnh ton c trn ta c th v c biu ph tr nh sau:

f1 f2BIU B TR

S dung biu tnh ton iu tit

Theo trn ta tnh c dung tch hiu dng ca h cha :

vh =v~ =153.38x106(m3)

=>VB, = vh + vc =153.38 + 7.5 = 160.88x106(tm3)

Da vo quan h z ~ V ta xc nh c mc nc ang bnh thng Hbt = 45.54 (m) => Mc nc trc khi l v l Zti = Hbt = 45.54 (m).

chnh lch mc nc gia mc nc trc l v ngng trn l: h = Hbt - Ztr = 45.54 - 41.2 =4.24 (m).

=> Lu lng nc x u thi on l:

q\ =mxbx^xgxh312 =0.49x24x V2 x 9.81 x4.243/2 = 454.78(w3/i)

Tip theo, xc nh Vsc v hsc ta phi xc nh c ng qu trnh x l bng cch lp bng tnh nh sau:

At

(h)

Q

(m3/s)

Qtb

(m3/s)

ql

(m3/s)

fl(q)

f2(q)

q2

(m3/s)

qtb

(m3/s)

AW

(106m3)

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

0.0

0.00

2.8

83.94

41.9722

454.78

8084.72

8126.70

423.23

439.005

-4.002

2.8

687.30

385.62

423.23

7597.93

7983.55

414.51

418.873

-0.335

2.8

1395.58

1041.44

414.51

7462.42

8503.86

446.35

430.434

6.159

2.8

2775.41

2085.49

446.35

7955.26

10040.75

542.92

494.639

16.036

2.8

3777.50

3276.46

542.92

9414.18

12690.63

718.31

630.616

26.670

2.8

2292.73

3035.12

718.31

11925.38

14960.50

877.47

797.89

22.551

2.8

1259.17

1775.95

877.47

14049.95

15825.90

940.33

908.899

8.740

2.8

1075.54

1167.35

940.33

14848.47

16015.82

954.28

947.303

2.218

2.8

891.91

983.724

954.28

15022.64

16006.36

953.58

953.931

0.300

2.8

718.77

805.342

953.58

15013.97

15819.31

939.84

946.713

-1.425

2.8

577.12

647.946

939.84

14842.42

15490.36

915.81

927.828

-2.821

2.8

439.66

508.389

915.81

14539.78

15048.17

883.79

899.799

-3.945

2.8

360.44

400.048

883.79

14131.19

14531.24

846.74

865.263

-4.689

2.8

308.50

334.466

846.74

13651.18

13985.64

808.10

827.422

-4.969

2.8

273.87

291.182

808.10

13142.08

13433.26

769.47

788.789

-5.016

2.8

229.80

251.833

769.47

12624.39

12876.22

731.01

750.243

-5.024

2.8

219.83

224.814

731.01

12100.36

12325.17

693.45

712.231

-4.913

2.8

199.37

209.599

693.45

11580.33

11789.93

657.43

675.442

-4.696

2.8

179.43

189.4

657.43

11073.98

11263.38

622.45

639.941

-4.541

2.8

174.71

177.07

622.45

10574.94

10752.01

588.90

605.672

-4.320

2.8

169.99

172.348

588.90

10089.67

10262.02

557.14

573.017

-4.039

2.8

174.71

172.348

557.14

9624.35

9796.70

527.34

542.238

-3.728

Tng cng

24.209

Trong :

Ct (1)l thi onquan trc, y ta ly bng t = 2.8 (h).Ct (2)l lu lngl n (ly theo qu trnh l thit k tnh c trn).Ct (3)l lu lngl n bnh qun thi on Qi - ^ 2 '+1Ct (4)l lu lngnc x u thi on qlCt (5)l gi tr fl(q) tnh c da trn biu quan h fl(q)~qxy

dng trn ( ct ny, ta tra gi tr fl(q) theo ql).

Ct (6) l gi tr f2(q) tnh c qua cng thc f2(q) = fl(q) + Q.Ct (7) l lu lng nc x cui thi on xc nh c thng qua biu quan h f2(q) ~ q vi gi tr f2(q) xc nh trn.Ct (8) l gi tr lu lng x trung bnh tnh theo cng thc: