Answer:
The bracket is the binomial.
The definition is:
Note that
The meaning is "if we have n elements, how many ways are there to choose k of them?" From this definition it should be obvious that
- if you're picking k, it's just as many ways as choosing the (n-k) that you're not picking.
The coefficient can be found by realizing that to get x^9, we need to choose (from the full multiplication):
- 9 brackets to provide x
- 2 brackets to provide the number 3
There's
ways to do that, so the component for x^9 will be:
So:
a = 2
Coefficient is 495