Question

若x-y=2,x^2-y^2=10,则3x-y

Answer 1

x^2-y^2
=(x-y)(x+y)=10
又已知x-y=2.......(1)
2(x+y)=10
x+y=5.........(2)

(1)+(2)得
2x=7
x=3.5
代入(2)得
3.5+y=5
y=1.5

解得
x=3.5
y=1.5

则
3x-y
=10.5-1.5
=9

Answer 2

联立两个式子
解得
把x=7/2 y=3/2代入3x-y中
解得3x-y=3×7/2-3/2=9

Answer 3

x=y+2 (y+2)^2-y^2=10 y=3/2 x=7/2
3x-y=9