Question

What is the numerical coefficient of the a^8*b^2 term in the expansion of ((1/3)a^2 - 3b)^6 ?

Answer 1

In the binomial development, the main problem is calculation of binomial coefficients.

If we want to get term a∧8*b∧2 we see that this is the third member in binomial development (n 2) a∧n-2*b∧2

The given binomial  is ((1/3)a∧2 - 3b)∧6, the first element is (1/3)a∧2, the second element is (-3b) and n=6 when we replace this in the formula we get

(6 2) * ((1/3)a∧2)∧(6-2) * (-3b)2 = (6*5)/2 * ((1/3)a∧2)∧4 *9b∧2= 15*(1/81)*9 *(a∧8b∧2) =

= 15*9* a∧8b∧2 = 135*a∧8b∧2

We finally get numerical coefficient 135

Good luck!!!