Toán nâng cao 8

Question

tính :

(x+1)(x+2)(x+3)(x+4) - 15

Answer 1

tính :

(x+1)(x+2)(x+3)(x+4) - 15

=[(x+1).(x+4)].[(x+2).(x+3)]-15

=(x²+5x+4)(x²+5x+6)-15

=(x²+5x+5-1)(x²+5x+5+1)-15

gọi x²+5x+5=a

=> (a-1)(a+1)-15

=a²-1-15

=a²-16

=(a-4)(a+4)

=(x²+5x+1)(x²+5x+9)