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# C2 - Solving trig equation mistake

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1. This question is from C2 Edexcel June 2013, Q8 part (ii) (b)

I let cos theta = x for typing faster.

4x^2 + 2x -1 = 0

I used completing the square

4(x^2+x/2)-1=0

4(x+1/4)^2-(1/16)-1=0

4(x+1/4)^2=17/16

(x+1/4)^2=17/64

(x+1/4)= root (17/64)

x=-1/4 plus or minus root (17/64)

That gives x, or cos theta, = 0.265.... or =-0.265....

This isn't correct, can anyone please tell me what I've done wrong?
2. (Original post by jamb97)
This question is from C2 Edexcel June 2013, Q8 part (ii) (b)

I let cos theta = x for typing faster.

4x^2 + 2x -1 = 0

I used completing the square

4(x^2+x/2)-1=0

4(x+1/4)^2-(1/16)-1=0

4(x+1/4)^2=17/16
Bolded bits. You should review completing the square again or how to expand brackets.

This gives
3. (Original post by Zacken)
Bolded bits. You should review completing the square again or how to expand brackets.

This gives
Woops... Got it. Thanks very much
4. (Original post by jamb97)
Woops... Got it. Thanks very much
Using big brackets when you have nested expressions like these are very useful for avoiding these types of mistakes.
5. (Original post by jamb97)
-
Alternatively you could just use the quadratic formula to avoid silly mistakes.
6. (Original post by edothero)
Alternatively you could just use the quadratic formula to avoid silly mistakes.
Or make more silly mistakes using the quadratic formula.
7. (Original post by Zacken)
Or make more silly mistakes using the quadratic formula.
I can relate to that statement
8. (Original post by edothero)
I can relate to that statement
Me 2 years ago in GCSE:

2 kul for u bby imma use quadratic formula

#negative discriminant

imaginary numbers betch
9. (Original post by Zacken)
Me 2 years ago in GCSE:

2 kul for u bby imma use quadratic formula

#negative discriminant

imaginary numbers betch
lmaooo I remember once when I was ill I had a c3 mock, started using complex numbers thinking I was doing fp2. Facepalms were real
10. (Original post by edothero)
lmaooo I remember once when I was ill I had a c3 mock, started using complex numbers thinking I was doing fp2. Facepalms were real

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