Question

Write the 5th term in the expansion of (a+b)⁷

Answer 1

Answer:

(70)(a)7+(71)(a)6(b)+(72)(a)5(b)2+(73)(a)4(b3)+(74)(a)3(b4)+…=(70)(a)7+(71)(a)6(b)+(72)(a)5(b)2+(73)(a)4(b3)+(74)(a)3(b4)+…

=(1)(a7)+(7)(a6)(b)+(21)(a5)(b2)+(35)(a4)(b3)+(35)(a3)(b4)+…=(1)(a7)+(7)(a6)(b)+(21)(a5)(b2)+(35)(a4)(b3)+(35)(a3)(b4)+…

=a7+7a6b+21a5