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Year 2 (A-level) Partial Fractions and Algebraic Fractions

Im stuck on a question because I'm not too sure about how to approach it...

Q. Find the values of the constants A, B, C, D and E in the following identity:
5x^4 - 4x^3 + 17x^2 - 5x + 7 = (Ax^2 + Bx +C)(x^2 + 2) + Dx + E

Reply 1

Original post by TheMillenialNo.3
Im stuck on a question because I'm not too sure about how to approach it...

Q. Find the values of the constants A, B, C, D and E in the following identity:
5x^4 - 4x^3 + 17x^2 - 5x + 7 = (Ax^2 + Bx +C)(x^2 + 2) + Dx + E



Standard technique would be to start with the RHS, expand the brackets and group like terms (same powers of x) together.

Then since this is an identity, the coefficients of a given power of x will be the same on the right and left of the identity.

This will give you a set of simultaneous equations to solve.

Reply 2

Original post by ghostwalker
Standard technique would be to start with the RHS, expand the brackets and group like terms (same powers of x) together.

Then since this is an identity, the coefficients of a given power of x will be the same on the right and left of the identity.

This will give you a set of simultaneous equations to solve.

So Ive expanded the RHS and got:
Ax^4 + Bx^3 + (2A + C)x^2 +(2 + 2B + D) + E

Am I going the right way?

Reply 3

Original post by TheMillenialNo.3
So Ive expanded the RHS and got:
Ax^4 Bx^3 (2A C)x^2 (2 2B D) E

Am I going the right way?


That’s the right method, so you can see that A must be 5. Can you find the others?
(edited 4 years ago)

Reply 4

Original post by TheMillenialNo.3
So Ive expanded the RHS and got:
Ax^4 + Bx^3 + (2A + C)x^2 +(2 + 2B + D) + E

Am I going the right way?


Your on the right lines, but you've made some errors there. x^4 down to x^2 terms are correct.

The x and constant terms have errors - I presume the missing x after the parentheses is just a typo.
In particular, where did the "2" by itself come from?
(edited 4 years ago)

Reply 5

Original post by ghostwalker
Your on the right lines, but you've made some errors there. x^4 down to x^2 terms are correct.

The x and constant terms have errors - I presume the missing x after the parentheses is just a typo.
In particular, where did the "2" by itself come from?

Im not sure where I got that 2 from. It was when I was expanding it and just misread my writing. I think ive got it now anyway. Thanks for the help.

Reply 6

Original post by TheMillenialNo.3
Im stuck on a question because I'm not too sure about how to approach it...
Q. Find the values of the constants A, B, C, D and E in the following identity:
5x^4 - 4x^3 + 17x^2 - 5x + 7 = (Ax^2 + Bx +C)(x^2 + 2) + Dx + E

pretty sure just dividing the rhs by (X^2 + 2) should give you everything in just one step. You'll get the A,B and C and the remainder Dx + E

Reply 7

Original post by archie.smith23
pretty sure just dividing the rhs by (X^2 + 2) should give you everything in just one step. You'll get the A,B and C and the remainder Dx + E

Please check the date on the post before replying - your answer is 3 years too late for the OP :smile:

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