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# Expansions in C2 watch

1. For expansions in C2 can you use the the C4 method? or does it depend?
2. (Original post by Spartcus)
For expansions in C2 can you use the the C4 method? or does it depend?
Yes you can use either of the binomial expansion results, though questions are usually designed so you don't need to.

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3. (Original post by gdunne42)
Yes you can use either of the binomial expansion results, though questions are usually designed so you don't need to.

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Thank you!
4. (Original post by Spartcus)
Thank you!
The expansion used in A2 can be especially useful in AS if you are solving for an unknown value of the power of the binomial.

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5. (Original post by gdunne42)
The expansion used in A2 can be especially useful in AS if you are solving for an unknown value of the power of the binomial.

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Sorry what do you mean by this?
6. (Original post by Spartcus)
Sorry what do you mean by this?
7. The first four terms, in ascending powers of x, of the binomial expansion of (1 + kx)^n are

1 + Ax + Bx^2 + Bx^3 + …, where k is a positive constant and A, B and n are positive integers.

(a) By considering the coefficients of x^2 and x^3, show that 3 = (n – 2) k.
(4)
Given that A = 4, (b) find the value of n and the value of k.
(4)

from the original C2 mock paper

As you don't know n, you can't just hit the nCr button
7. (Original post by gdunne42)
7. The first four terms, in ascending powers of x, of the binomial expansion of (1 + kx)^n are

1 + Ax + Bx^2 + Bx^3 + …, where k is a positive constant and A, B and n are positive integers.

(a) By considering the coefficients of x^2 and x^3, show that 3 = (n – 2) k.
(4)
Given that A = 4, (b) find the value of n and the value of k.
(4)

from the original C2 mock paper

As you don't know n, you can't just hit the nCr button
I see, makes sense. But in pretty much everything else is it okay to use the c4 expansion method?
8. (Original post by Spartcus)
I see, makes sense. But in pretty much everything else is it okay to use the c4 expansion method?
Yes. With Edexcel, most C2 questions are most easily done using the (a+b)^n expansion but all can be be done with the (1+x)^n expansion if you factorise out the a correctly.Either method correctly implemented earns full marks.
9. (Original post by gdunne42)
7. The first four terms, in ascending powers of x, of the binomial expansion of (1 + kx)^n are

1 + Ax + Bx^2 + Bx^3 + …, where k is a positive constant and A, B and n are positive integers.

(a) By considering the coefficients of x^2 and x^3, show that 3 = (n – 2) k.
(4)
Given that A = 4, (b) find the value of n and the value of k.
(4)

from the original C2 mock paper

As you don't know n, you can't just hit the nCr button
How come they've put this in a C2 mock? Surely the A2 method isn't known to AS students?
10. (Original post by NotKidding)
How come they've put this in a C2 mock? Surely the A2 method isn't known to AS students?
It is mentioned in the C2 edexcel reveision book, you have to know both techniques. But for an A2 student, it would be easier if he could just use the 1 + nx + technique as it is more familiar, hence why i have asked, as I am resitting this week.
11. (Original post by NotKidding)
How come they've put this in a C2 mock? Surely the A2 method isn't known to AS students?
Both expansions are expected to be studied in AS. There isn't an A2 method it's just that only the (1+x)^n method works for fractional and negative powers studied at A2

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