Ta có:
A= \(\left(a+1\right)\left(a+3\right)\left(a+5\right)\left(a+7\right)+15\)
= \(\left[\left(a+1\right)\left(a+7\right)\right].\left[\left(a+3\right)\left(a+5\right)\right]+15\)
= \(\left(a^2+8a+7\right).\left(a^2+8a+15\right)+15\)
Đặt \(t=a^2+8a+11\)
\(\Rightarrow\) A= \(\left(t-4\right)\left(t+4\right)\) +15
= \(t^2-16+15\)
= \(t^2-1\)
= \(\left(t+1\right)\left(t-1\right)\)
= \(\left(a^2+8a+11+1\right)\left(a^2+8a+11-1\right)\)
= \(\left(a^2+8a+10\right)\left(a^2+8a+12\right)\)
Chúc bạn học tốt. 
\(A=\left(a+1\right)\left(a+3\right)\left(a+5\right)\left(a+7\right)+15\)
\(=\left(a+1\right)\left(a+7\right)\left(a+3\right)\left(a+5\right)+15\)
\(=\left(a^2+8a+7\right)\left(a^2+8a+15\right)+15\)
\(=\left(a^2+8a+11-4\right)\left(a^2+8a+11+4\right)+15\)
\(=\left(a^2+8a+11\right)\left(a^2+8a+11\right)-4.4+15\)
\(=\left(a^2+8a+11\right)^2-1\)
A=(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
Đặt a2+8a+11=t
=>(t-4)(t+4)+15
=t2+4t-4t-16+15
=t2-16+15
=t2-1
=(t+1)(t-1)
=(a2+8a+11+1)(a2+8a+11-1)
=(a2+8a+12)(a2+8a+10)
