1.(x+1)*(x+2)*(x+3)*(x+4)-120 =(x+1)(x+4)(x+2)(x+3)-120=(x^2+5x+4)(x^2+5x+6)-120
设t=x^2+5x则原式=(t+4)(t+6)-120=t^2+10t+24-120=t^2+10t-96(十字相乘)=(t+16)(t-6)=(x^2+5x+16)(x^2+5x-6)=(x^2+5x+16)(x+6)(x-1)
2..(x-1)*(x-3)*(x+1)*(x+3)-20=(x^2+3-4x)(x^2+3+4x)-20=(x^2+3)^2-16x^2-20=x^4+6x^2+9-20-16x^2=x^4-10x^2-11=(x^2-11)(x^2+1)
=(x-根号11)(x+根号11)(x^2+1)