(x^2+4x+3)(x^2+12x+35)+15
=（x+1)(x+3)(x+5)(x+7)+15
=(x^2+8x+7)(x^2+8x+15)+15
设t=x^2+8x+11,
上式=（t-4)(t+4)+15
=t^2-16+15
=(t-1)(t+1)
=(x^2+8x+10)(x^2+8x+12)
=(x+4+√6）（x+4-√6）（x+2)(x+6).