p) ( 38 x 35 – 19 x 70) x ( 1 x 2 + 2 x 3 + 3 x 4 +...+ 9 x 10)
= ( 38 x 35 – 19 x2 x 35)x(1 x 2 + 2 x 3 + 3 x 4 + 4 x 5 + ... +9 x 10)
= ( 38 x 35 - 38 x 35) x ( 1 x 2 + 2 x 3 + 3 x 4 + 4 x 5+ ... + 9 x 10)
= [38 x (35-35)] x ( 1 x 2 + 2 x 3 + 3 x 4 + 4 x 5 + ... + 9 x 10)
= (38 x 0) x ( 1 x 2 + 2 x 3 + 3 x 4 + 4 x 5+ ... + 9 x 10)
= 0 x ( 1 x 2 + 2 x 3 + 3 x 4 + 4 x 5+ ... + 9 x 10)
=0 ( 0 x bao nhiêu cũng = 0)