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# C2 Binomial Expansion Question 1 watch

1. I can't figure out where I'm going wrong here...

If (1 + 2x)^5 + (1 - 2x)^5 = a + bx + cx^2,
find the values of the constants a,b and c.

I used binomial expansion and I got:
(1 + 2x)^5 + (1 - 2x)^5 =

1 + 10x + 40x^2 + 80x^3 + 80x^4 + 32x^5 + 1 - 10x + 40x^2 - 80x^3 + 80x^4 - 32x^5 =

2 + 80x^2 + 160x^4

2. I agree - question seems to be wrong.
3. (Original post by Narik)
I can't figure out where I'm going wrong here...

If (1 + 2x)^5 + (1 - 2x)^5 = a + bx + cx^2,
find the values of the constants a,b and c.

I used binomial expansion and I got:
(1 + 2x)^5 + (1 - 2x)^5 =

1 + 10x + 40x^2 + 80x^3 + 80x^4 + 32x^5 + 1 - 10x + 40x^2 - 80x^3 + 80x^4 - 32x^5 =

2 + 80x^2 + 160x^4

Edit: Your calculation is correct but the question only asks for the constant term and the coefficients of and
4. (Original post by Narik)
I can't figure out where I'm going wrong here...

If (1 + 2x)^5 + (1 - 2x)^5 = a + bx + cx^2,
find the values of the constants a,b and c.

I used binomial expansion and I got:
(1 + 2x)^5 + (1 - 2x)^5 =

1 + 10x + 40x^2 + 80x^3 + 80x^4 + 32x^5 + 1 - 10x + 40x^2 - 80x^3 + 80x^4 - 32x^5 =

2 + 80x^2 + 160x^4

your working is correct, question seems wrong
5. Just adding voice to the question being wrong in this instance.
6. Unless you square rooted, then you would have a+bx+cx^2 but I dont remember having to do anything like that in C2.
7. (Original post by mcp2)
Unless you square rooted, then you would have a+bx+cx^2 but I dont remember having to do anything like that in C2.
No, the square root of 2+80x^2+160x^4 isn't even a polynomial, let alone one of the form a+bx+x^2.
8. Make x^2 x, would that work?
9. (Original post by mcp2)
Make x^2 x, would that work?
Not really. You're getting into "2+2 = 6" if we make 2 = 3 territory. Far simpler to assume the question's wrong, to be honest.
10. (Original post by DFranklin)
Not really. You're getting into "2+2 = 6" if we make 2 = 3 territory. Far simpler to assume the question's wrong, to be honest.
Why is the question wrong? It asks for certain coefficients , why does it matter that the question does not ask for the coefficient of
11. It doesn't ask for certain coefficients, it asks for the values of a,b,c such that (1 + 2x)^5 + (1 - 2x)^5 = a + bx + cx^2.

There are no such values, so the question is wrong.
12. (Original post by DFranklin)
It doesn't ask for certain coefficients, it asks for the values of a,b,c such that (1 + 2x)^5 + (1 - 2x)^5 = a + bx + cx^2.

There are no such values, so the question is wrong.
Thanks.

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