請問3個unknown點slove?

16A + 20B + 40C = 5 ...... (1)
20A + 100B + 70C = 7 ...... (2)
40A + 70B + 196C = 13 ...... (3)

(2) x 2:
40A + 200B + 140C = 14 ...... (4)

(4) - (3):
130B - 56C = 1
390B - 168C = 3 ...... (5)

(1) x 5:
80A + 100 B + 200C = 25 ...... (6)
(2) x 4:
80A + 400B + 280C = 28 ...... (7)
(7) - (6):
300B + 80C = 3 ...... (8)

(5) = (8):
390B - 168C = 300B + 80C
90B = 248C
B = (124/45)C ...... (9)

(9) 代入 (8) 中：
300(124/45)C + 80C = 3
37200C + 3600C = 135
C = 135/40800
C = 0.0033

把 C = 135/40800 代入 (9) 中：
B = (124/45)(135/40800)
B = 372/40800
B = 0.0091

把 C = 135/40800 和 B = 372/40800 代入 (1) 中：
16A + 20(372/40800) + 40(135/40800) = 5 ...... (1)
A = 0.2928

答：A = 0.2928，B = 0.0091，C = 0.0033。
=

2009-05-12 00:10:06 補充：
Check:
(1):
L.S = 16(0.2928) + 20(0.0091) + 40(0.0033) = 4.9988 ≈ 5 = R.S.

(2):
L.S = 20(0.2928) + 100(0.0091) + 70(0.0033) = 6.997 ≈ 7 = R.S.

(3):
L.S = 40(0.2928) + 70(0.0091) + 196(0.0033) = 12.9958 ≈ 13 = R.S.

The differences are due to rounding off in calculations.

(16A + 20B + 40C)-(20A + 100B + 70C)=5-7
-4A-80B-30C=-2
4A+80B+30C=2
(40A + 70B + 196C)-(20A + 100B + 70C)=13-7
20A-30B+126C=6B+
(20A-30B+126C)-(4A+80B+30C)=6-2
16A-110B-96C=4

(16A + 20B + 40C)-(16A-110B-96C)=5-4
130B+136C=1
130*0.0091+136*0.0033not=1
soB is not =0.0091,Cis not =0.0033

2009-05-13 20:08:25 補充：
The answer must be excatly correct
I give you a example:1+0.99=2
Can we say that?
Of course no!
But you can say:1+0.99≈2
So,your anwser is wrong because you say 16A + 20B + 40C is excatly =5.
But16(0.2928) + 20(0.0091) + 40(0.0033) = 4.9988,
so the answer is not excatly = 5.

2009-05-13 20:23:30 補充：
So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong So,your anwser is wrong

3 個unknowns
to
2 個unknowns
to
1 個unknown
then
"solve"