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Addition formula

How would I do question 7 both parts, I have no notes on how to do it and don't really know where to begin
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Original post by Jism1495
How would I do question 7 both parts, I have no notes on how to do it and don't really know where to begin


Do you know the formulae for sin (A+B) and cos(A+B), if so use them remembering that 2x = x + x
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B_9710
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Original post by Jism1495
How would I do question 7 both parts, I have no notes on how to do it and don't really know where to begin


Use the double angle formula for the first one. Then if sin2P=24/25
Draw a right triangle with angle 2P where the opposite to the angle 2P is 24 and the hypotenuse is 25. Use Pythagoras to find the adjacent and remember that cos2P=adjacent/hypotenuse
Original post by B_9710
Use the double angle formula for the first one. Then if sin2P=24/25
Draw a right triangle with angle 2P where the opposite to the angle 2P is 24 and the hypotenuse is 25. Use Pythagoras to find the adjacent and remember that cos2P=adjacent/hypotenuse


Although that would work in this situation, care needs to be taken. If sinP had been 4/5 it would fail, as 2P would be greater than 90, and without checking you'd end up with cos2P being positive, when in fact it's negative.

Using the formula for cos2P would bypass this problem.
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B_9710
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Original post by ghostwalker
Although that would work in this situation, care needs to be taken. If sinP had been 4/5 it would fail, as 2P would be greater than 90, and without checking you'd end up with cos2P being positive, when in fact it's negative.

Using the formula for cos2P would bypass this problem.


Of course but if you take a second to think about it when you find the cos2P value you should be able to decide if it is positive or negative. But you're right care does need to be taken

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