-(-x^2/2a) 不 改变分式的值,使分式分子和分母都不带负号.（3x^2+x)÷(x^2-x),用分时表示商,并约分约分：1.y-x/x^2-y^2 2.2x^2-10x/x^2-10x+25 3.a^2+6a+9/a^2-9

-(-x^2/2a) =x²/2a
（3x^2+x)÷(x^2-x)=x(3x+1)÷x(x-1)=(3x+1)/(x-1)
约分：
1.y-x/x^2-y^2=(y-x)/(x-y)(x+y)=-1/(x+y)
2.2x^2-10x/x^2-10x+25=2x(x-5)/(x-5)²=2x/(x-5)
3.a^2+6a+9/a^2-9=(x+3)²/(x-3)(x+3)=(x+3)/(x-3)