(a+1)(a+3)(a+5)(a+7)+15请写出具体过程,或者文字解释,

(a+1)(a+3)(a+5)(a+7)+15=(a+1)(a+7)(a+3)(a+5)+15=(a²+8a+7)(a²+8a+15)+15设a²+8a+11=A(a+1)(a+3)(a+5)(a+7)+15=(A-4)(A+4)+15=A²-16+15=A²-1=(A+1)(A-1)=(a²+8a+12)(a²+8a+10)...