Đặt A=(x2+8x+7).(x+3).(x+5)+15
A=(x2+8x+7)(x2+8x+15)+15
Đắt t=x2+8x+7
=>A=t(t+8)+15
<=>A=t2+8t+15
<=>A=t2+3t+5t+15
<=>A=t(t+3)+5(t+3)
<=>A=(t+3)(t+5)
=>A=(x2+8x+10)(x2+8x+12)
=>A=(x2+8x+10)(x2+2.4.x+16-22)
=>A=(x2+8x+10)(x+4-2)x+4+2)
=>A=(x2+8x+10)(x+2)(x+6)