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Polynomials help

f(x)=2x^2+px+q.

Given that f(-3) = 0 and f(4) = 21:Find value of p and q

So i formed 2 equations.

32 + 4p + q = 21 A
18 - 3p + q = 0 B

Rearranged to get:

q = 3p - 18
q = -4p - 11

3p - 18 = -4p - 11

Eliminated q to get p = 1

Is this correct?
(edited 11 years ago)
Reply 1
Avatar for Deziah
Deziah
15
You could put all the numbers on one side and treat them like simultaneous equations. :smile:
Reply 2
Avatar for Prendah
Prendah
1
Haven't checked if it is correct, but with your expression for q you can sub it into the other equation and find out p.
Reply 3
Avatar for usycool1
usycool1
19
Original post by Tynos
f(x)=2x^2+px+q.

Given that f(-3) = 0 and f(4) = 21:Find value of p and q

So i formed 2 equations.

32 + 4p + q = 21 A
18 - 3p + q = 0 B

Rearranged to get:

q = 3p - 18
q = -4p - 11

3p - 18 = -4p - 11

Eliminated q to get p = 1

Is this correct?


Yes, that's right. Now sub that value back into one of the equations to get the value of q. :smile:
Reply 4
Avatar for Tynos
Tynos
OP
2
Well for some apparent reason p = -1 in the book.
Reply 5
Avatar for usycool1
usycool1
19
Original post by Tynos
Well for some apparent reason p = -1 in the book.


The books answer is wrong then. :smile:
Reply 6
Avatar for Tynos
Tynos
OP
2
DOnt the examiners like check the answers first? -.-

Its Edexcel;.

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