二元一次（maths） quickly!!

(1/x) + (1/y) = 0.25 .............. (A)
(1/x) + 2(1/y) = 0.75 ............. (B)

(B) - (A):　(1/y) = 0.5
　　　　　　y = 2.

Put into (A):　(1/x) + 0.5 = 0.25
　　　　　　　　 (1/x) = -0.25
　　　　　　　　　　x = -4.

2007-03-18 15:50:02 補充：
其實你把(1/x)當成a, (1/y)當成b, 那麼兩條等式就變成一般的二元一次等式。

1\x+1\y=1/4
1\x+2\y=3/4

let a and b be 1/x and 1/y respectively
a+b=1/4---(1)
a+2b=3/4---(2)

(2)-(1)
b=1/2
sub b=1/2 into (1)
a+(1/2)=1/4
a= -1/4

a= -1/4 , b= 1/2
(actually the above calculation can be solved by the calculator)


AS a= -1/4 , b= 1/2
so 1/x= -1/4
x= -4
then 1/y =1/2
y =2

CHECK:
1\x+1\y=0.25
1\x+2\y=0.75

1\(-4)+1\(2)=0.25 correct
1\(-4)+2\(2)=0.75 correct

1\x+1\y=0.25X
1\x+2\y=0.75
1\x15% 2\y60%