Posted by **kathleen** on Monday, December 6, 2010 at 3:50pm.

I am stuck on a problem that has to do with the double angle formulas. I have to solve this: 4sinxcosx = 1. I tried using the formula 2sinu = 2sinucosu by substituting it in like this: 2(2sinxcosx)= 1 but then I am stuck

- Math -
**Reiny**, Monday, December 6, 2010 at 4:51pm
remember that sin 2x = 2sinxcosx

so 4sinxcosx = 1

2(2sinxcosx) = 1

2(sin2x) = 1

sin2x = 1/2

I will assume you are working in degrees, we can always switch later on

so 2x = 30° or 2x = 150° because of the CAST rule

x = 15° or x = 75°

the period of sin 2x is 180°, so adding 180 to any answer will produce new answers

x = 15°, 195°, ...

or

x = 75°, 255°, ...

In Radians:

15° = π/12 radians

x = π/12, 13π/12....

x = 5π/12, 17π/12, ...

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