m-15/m²-9－2/3-m =（m-15）/(m+3)(m-3) +2(m+3)/(m+3)(m-3)
=3(m-3)）/(m+3)(m-3)=3/(m+3)
(x-1－8/x+1)÷x+3/x-1这个实在分辨不来 是按此计算
[x-1-8/(x+1)]÷(x+3)/(x-1）
={[x(x+1)-(x+1)-8]/(x+1)}÷(x+3)/(x-1)=[(x+3)(x-3)/(x+1)]*(x-1)/(x+3)
=(x-3)(x-1)/(x+1)
1/4x²-9y²÷(1/2x+3y－1/ 2x-3y)=1/(4x²-9y²)÷[1/(2x+3y)-1/(2x-3y)]
=1/(2x+3y)(2x-3y)÷[(2x-3y)-(2x+3y)]/(2x+3y)(2x-3y)
=-1/6y