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# C4 Trigonometry (Proof) watch

1. I need assistance with the proof question (9(i)). The others don't have any trouble with, only the first part. I'm capable with double angle formulae and addition formulae, just seem to be going in circles. Thanks for any help.
2. (Original post by spmenz123)

I need assistance with the proof question (9(i)). The others don't have any trouble with, only the first part. I'm capable with double angle formulae and addition formulae, just seem to be going in circles. Thanks for any help.
3. (Original post by spmenz123)

I need assistance with the proof question (9(i)). The others don't have any trouble with, only the first part. I'm capable with double angle formulae and addition formulae, just seem to be going in circles. Thanks for any help.
Start with the LHS, expand your double angles and theta+45, and it drops out in a few lines. Careful with the signs.

Post some working if you'd like us to check what you've done.
4. (Original post by ghostwalker)
Start with the LHS, expand your double angles and theta+45, and it drops out in a few lines. Careful with the signs.

Post some working if you'd like us to check what you've done.
Im assuming its correct to use addition formulae when its cos^2(a+b) and substitute each cosx and sinx with cos^2x and sin^2x?

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5. (Original post by spmenz123)
Im assuming its correct to use addition formulae when its cos^2(a+b) and substitute each cosx and sinx with cos^2x and sin^2x?

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'Fraid not.

which works out differently to what you have due to the cross terms.
6. (Original post by ghostwalker)
'Fraid not.

which works out differently to what you have due to the cross terms.
Ah theres my error. Thanks!

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7. Alternatively you could replace the squared sines and cosines and similar for the sine, .
8. I was stuck on question 9, I've did the first part after agess but could you tell me how you did the second part?
9. (Original post by asdfaeth)
I was stuck on question 9, I've did the first part after agess but could you tell me how you did the second part?
Look at how the LHS is different on (ii) than to (i), its been multiplied by 6 and θ has been multiplied by 1/2, therefore 6sin^2θ/2=2. Solve from here to the constraints.

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10. thank youu

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