help finding terms

Question

 

 

1.  write the first three terms of the expansion of (a - b)^6

 

 

2.  what are the last three terms of (a - b)^7

 

thanks

Answer 1

(a - b)6=a⁶-6a⁵b+((6×5)/(1×2))a⁴b²-((6×5×4)/(1×2×3))a³b³+((6×5)/(1×2))a²b⁴-6ab⁵+b⁶

take the first three terms to the right of the equal sign

 

(a-b)⁷=Σ_k=0^k=7₇C_ka^k(-b)^7-k the last three terms are for k=5,6,7