Originally Posted by

**TomCat** Hi,

I'm getting close to the right answer, but one little thing is off. I think I'm missing a negative somewhere...

Here's my work:

I make a couple of pictures to find the missing side. For alpha I get a triangle in quadrant II. I get the missing, base, side is equal to 12 after using the Pythagorean theorem. The 12 is also negative because of the quadrant.

For beta I get another triangle in quadrant II. I get the missing, hypotenuse, side is equal to 2.

I then plug the numbers into the sum and difference formula for Sin(alpha + beta) which is sin alpha cos beta + cos alpha sin beta

I get (5/13) (-1/2) + (-12/13) (sqrt3/2)

= (-5/26) + (-12sqrt3/26)

= -5 - 12sqrt3 all over 26.

The answer should be 5 + 12sqrt3 all over 26.

Where are my negatives coming from? Did I use the wrong quadrant or did I miss something in the work?

Thanks,

-Tom