A=(a+1)(a+3)(a+5)(a+7)+15
A=[(a+1)(a+7)][(a+5)(a+3)]+15
A=(a2+8a+7)(a2+8a+15)+15
Đặt a2+8a = v
Ta có :
A=(v+7)(v+15)+15
A= v2+22v+105+15
A= v2+22v+ 120
A= v2+10v+12v+120
A=( v2+10v)+(12v+120)
A=[v(v+10)]+[12(v+10)]
A=(v+10)(v+12) (1)
Thay a2+8a = v vào (1)
A=(a2+8a+10)(a2+8a+12)