1.x+1分之1÷（x+1分之1+x平方-1分之1）
1/(x+1) ÷ { 1/(x+1) + 1/(x^2-1) }
= 1/(x+1) ÷ { (x-1+1)/(x^2-1) }
= 1/(x+1) ÷ x/{(x+1)(x-1) }
= 1/(x+1) ×(x+1)(x-1) /x
= (x-1) /x
2.a-b分之a + a（b-a）分之b平方
a/(a-b) + b^2/{a(b-a)}
= a/(a-b) - b^2/{a(a-b)}
= (a^2 - b^2)/{a(a-b)}
= (a+b)(a-b)/{a(a-b)}
= (a+b)/a
3.已知a+b=2.ab=-5
(1) a分之1+b分之1
=(a+b)/(ab) = 2/(-5) = -2/5
(2)b分之a+a分之b
=(a^2+b^2)/(a+b) = {(a+b)^2-2ab}/(a+b) = {2^2-2*(-5)}/2 = 14/2 = 7