Question

有一道题先化简再求值:(x-2/x+2)+(4x/x^2-4)除以(1/x^2-4),其中x=200.小玲做题时把x=2

Answer 1

[(x-2/x+2)+(4x/x^2-4)]÷(1/x^2-4)

=[(x-2)/(x+2)+(4x/x^2-4)]×(x^2-4)

=(x-2)/(x+2)×(x^2-4)+(4x/x^2-4) ×(x^2-4)

=(x-2)²+4x

=x²+4,这样将x=20代入进去就ok了