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# need help on this C3 question watch

1. using sin(θ-a) = sinθcosa - cosθsina, or otherwise, show that:
sin 15 = 1/4 (square root of 6 - square root of 2)

I don't understand how to tackle this question

2. find two values x and y such that x - y = 15, then substitute into the identity
3. Chose 2 nice values for Sin(x-y) though like umm 45 and 30
4. (Original post by Snapshot13)
Chose 2 nice values for Sin(x-y) though like umm 45 and 30
yh use 45 and 30 and work out each value and you should get it.
5. so just sub in like 50 and 35 into sinθcosa - cosθsina?

but how then do I get to 'sin 15 = 1/4 (square root of 6 - square root of 2)'......presumbly thats what Im meant to do...
6. (Original post by W.H.T)
so just sub in like 50 and 35 into sinθcosa - cosθsina?

but how then do I get to 'sin 15 = 1/4 (square root of 6 - square root of 2)'......presumbly thats what Im meant to do...
You should know the values of sin 45, sin 30, cos 45 and cos 30. If you don't know them then use a reference triangle and Pythagoras to work them out.
7. (Original post by steve2005)
You should know the values of sin 45, sin 30, cos 45 and cos 30. If you don't know them then use a reference triangle and Pythagoras to work them out.
Oooor put them into your caculator if it tells you the exact (surds) value of them
8. (Original post by Snapshot13)
Oooor put them into your caculator if it tells you the exact (surds) value of them
yh but you would have to show working out
9. (Original post by victor_b1992)
yh but you would have to show working out
Realy? Coudent you just write
Sin30=.......
Cos30=.......

Sin45=....
Cos45=......

and the examiner would assume that you just knew the trig identys without having to draw down the triangles? :P

Sorry for my spelling
10. (Original post by Snapshot13)
Realy? Coudent you just write
Sin30=.......
Cos30=.......

Sin45=....
Cos45=......

and the examiner would assume that you just knew the trig identys without having to draw down the triangles? :P

Sorry for my spelling
haha C3 is calculator paper, so examiner obviously expect you to use it to your advantage whatever possible.
I just use the calculator to find value of sin(45 or 60) in surds form there is no need to waste time.
11. (Original post by Snapshot13)
Realy? Coudent you just write
Sin30=.......
Cos30=.......

Sin45=....
Cos45=......

and the examiner would assume that you just knew the trig identys without having to draw down the triangles? :P

Sorry for my spelling
If you know them then OK, but if you can't remember them and can't afford a decent calculator, then work them out using the reference triangles.
12. (Original post by steve2005)
If you know them then OK, but if you can't remember them and can't afford a decent calculator, then work them out using the reference triangles.
Thats what iv been waiting for my maths teacher to say allllll year :') He still wont admit that the triangles are pretty useless when you have a caculator which can do the job faster :P

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