# A level maths Edexcel Binomial Expansion

I'm stuck on part b)
g(x) = (2 + ax) ^8 where a is a constant
Given that one of the terms in the binomial expansion of g(x) is 3402x 5
(a) find the value of a. (4)

Using this value of a,
(b) find the constant term in the expansion of (1+ 1/x^4 ) (2 + ax) ^8 (3)

When working out part b) I get where the 256 came from but I don't get why you add 5670 to it to get a final answer of 5926????
I'm stuck on part b)
g(x) = (2 + ax) ^8 where a is a constant
Given that one of the terms in the binomial expansion of g(x) is 3402x 5
(a) find the value of a. (4)
Using this value of a,
(b) find the constant term in the expansion of (1+ 1/x^4 ) (2 + ax) ^8 (3)
When working out part b) I get where the 256 came from but I don't get why you add 5670 to it to get a final answer of 5926????

You are multiplying two brackets: (1 + 1/x^4), and (2 + ax)^8

The constant term will appear in two places of the expansion:

(1) when you multiply constant from first bracket with constant from second bracket,

and (2) when you multiply the 1/x^4 term from the first bracket with x^4 term from the second bracket.

Both constants in the end combine to give the final answer. Seems to me like you are missing the smaller constant arising from (2).
(edited 3 months ago)
Original post by RDKGames
You are multiplying two brackets: (1 + 1/x^4), and (2 + ax)^8
The constant term will appear in two places of the expansion:
(1) when you multiply constant from first bracket with constant from second bracket,
and (2) when you multiply the 1/x^4 term from the first bracket with x^4 term from the second bracket.
Both constants in the end combine to give the final answer. Seems to me like you are missing the smaller constant arising from (2).

Ohhhh!
Thank you! That makes sense now