Thread: Double Angle Forumla

The angle 2x lies in the fourth quadrant such that cos2x=8/17.

1.Which quadrant contains angle x?
2. Determine an exact value for cosx
3. What is the measure of x in radians?

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I know that quadrant 4 has 2x in it, so quadrant _____ has to have x ?
for part 2, the exact measure of cosx would, it be 4/8.5??? I divided 8 by two and 17 by two.. I dont know if it is right. Check?

For part 3, the measure of x, would I have to take cos^-1(4/8.5) [ if part b is right] to get the measure of x? Thanks!

Originally Posted by skeske1234

2. Determine an exact value for cosx
for part 2, the exact measure of cosx would, it be 4/8.5??? I divided 8 by two and 17 by two.. I dont know if it is right. Check?

No, is not the same as

The double angle formula states that .

Therefore , can you work this one out now?

Originally Posted by skeske1234

For part 3, the measure of x, would I have to take cos^-1(4/8.5) [ if part b is right] to get the measure of x?

No I'm afraid this is incorrect.

Let me give you an example, if for example where and are just numbers, then , therefore .

Can you work this one through?

As for the first part I'm afraid I'm unsure about the quadrant method.