Question

已知x不等于y,且x的平方减去x等于13,y的平方减去y等于13,求x+y-7的值

Answer 1

x+y-7=-6

x^2-x=13

y^2-y=13

两式相减

x^2-y^2-x+y=0

(x+y)(x-y)-(x-y)=0

(x-y)(x+y-1)=0

因为x≠y

所以x-y≠0

所以 x+y-1=0

x+y-7=(x+y-1)-6=-6