How do you find the ninth term of the expansion (a - 1)^13?
b a^13 + ... c a^12 + ...d a^11 etc
ninth term will have a^(13-8)-1^8 or a^5
so what is the coefficient?
(13,8) in Pascal triangle
13!/[8!(5!)]
13*12*11*10*9 / (5*4*3*2)
13* 11 *2 *9= 2574
2574a^5