Question

已知1/x+1/y=1/2,求代数式3x-5xy+3y/-x+3xy-y

Answer 1

1/x+1/y=1/2

(x+y)/xy=1/2

xy=2(x+y)

所以(3x-5xy+3y)/(-x+3xy-y)

=[3(x+y)-5*2(x+y)]/[-(x+y)+3*2(x+y)]

=-7(x+y)/[5(x+y)]

=-7/5