Question

(a+b)9 (-a-b)4 +(a+b)6 (a-b)7

Answer 1

应该是(a+b)9 (-a-b)4 +(a+b)6 (-a-b)7

=(a+b)9 [-(a+b)]4 +(a+b)6 [-(a+b)]7

=(a+b)9 (a+b)4 +(a+b)6 [-(a+b)7]

=(a+b)13-(a+b)13

=0