Posted by **Bailey** on Monday, January 11, 2010 at 9:12pm.

I can't remember what formula to use for these.

Note- the X's may be replaced by a theta symbol. I just didn't have one on my keyboard.

Use the given information to find sin2X, cos2X and tan2X if 0< X <pi/2

1. sinX=12/13

2. cosX=3/5

3. tanX=2/3

- Trigonometry -
**Diana**, Monday, January 11, 2010 at 9:37pm
You use the double angle formulas.

- Trigonometry -
**MathMate**, Tuesday, January 12, 2010 at 7:30am
To type θ, you need to type the following without the double quotes nor the spaces: "& t h e t a ;".

As Diana suggested, use the double angle formulas. I'll do the first one as an example.

sin(2θ)

=2sin(θ)cos(θ)

sin(&theta)=12/13

cos(&theta)=sqrt(1²-(12/13)²)=5/13

Therefore

sin(2θ)

=2sin(θ)cos(θ)

=2(12/13)(5/13)

=120/169

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