这个数是15两个质数相加的形式15=2+13两个合数相加的形式15=6+9质数与合数15=3+12=4+11=5+10=7+8
1/(x2-y2)=1/(x+y)(x-y)1/(x2+xy)=1/x(x+y)最简公分母x(x+y)(x-y)所以1/(x2-y2)=x/(x3-xy2)1/(x2+xy)=(x-y)/(x3-xy2)
