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# core 3 trigonometry watch

1. given that cosx=2/3 and x is acute, find the value of sin2x as a surd.
2. (Original post by hanbubbles21)
given that cosx=2/3 and x is acute, find the value of sin2x as a surd.
Go on then.

Have you attempted anything?
3. OK, here's a clue.

Draw a right angled triangle, (you might be able to do it by imagining the triangle, I always have to draw it) , make one of the angles x and use the fact that cosx = 2/3 to mark on two of the sides of the triangle. You can now calculate the 3rd side.

Then you can find what sin x is as well using opp/hyp and then all you need is your trig identities to find what sin 2x is.
4. sin(2x)=2sin(x)cos(x)

You know cos(x). You require sin(x).

Notice by drawing a right-angled triangle, you can use Pythogras.
5. (Original post by RDKGames)
Go on then.

Have you attempted anything?
hi yes i gave it a go and got an answer for sin2x to be 2. don't know if this is the correct answer but ill find out in my maths lesson tomorrow
6. (Original post by hanbubbles21)
hi yes i gave it a go and got an answer for sin2x to be 2. don't know if this is the correct answer but ill find out in my maths lesson tomorrow
You should know if it's correct or not. What's the range of the sine function?

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