通分 1.a/(a^2-9),a-1/a^2+6a+9 2.x/2(x+1),1/x^2-x 3.1/x^2-4,x/4

通分
1、a/(a^2-9)和(a-1)/a^2+6a+9,
a/(a^2-9)=a/(a+3)(a-3),
(a-1)(/a^2+6a+9) =(a-1)/(a+3)^2(a+3)=(a-1)/(a+3)(a+3),
公分母 (a+3)(a+3)(a-3),
即a/(a^2-9)=a/(a+3)(a-3)= a(a+3)/(a+3)(a+3)(a-3),
(a-1)(/a^2+6a+9) =(a-1)/(a+3)^2(a+3)=(a-1)/(a+3)(a+3)=(a-1)(a-3)/a+3)(a+3)(a-3).
2、x/2(x+1)和1/(x^2-x),
1/(x^2-x)=1/x(x-1),公分母为2x(x-1)(x+1),即
x/2(x+1) = x^2 * (x-1)/2x(x-1)(x+1),x^2为x的平方,
11/(x^2-x)=1/x(x-1)= 2(x+1)/2x(x-1)(x+1).
3、1/(x^2-4)和x/(4-2x),
1/(x^2-4) = 1/(x+2)(x-2),
x/(4-2x) = x/2(2-x),
公分母为2(x+2)(x-2),则
1/(x^2-4) = 1/(x+2)(x-2) = 2/2(x+2)(x-2),
x/(4-2x) = x/2(2-x) = - x(x+2)/2(x+2)(x-2)