college algebra word problem

Question

Use the Binomial Theorem to write the first three terms of the binomial.

(a + b)^¹⁵

Answer 1

a^15 + 15a^14b + 105a^13b^2 .....

a has decreasing powers

b has increasing powers

but for each term the powers sum to 15

with a power of 15, that's also the coefficient of the 2nd term