Originally Posted by

**guitargeek70** Hello all. For the last 2 hours I've been working on sum and difference problems and cannot seem to get answers that match the book's answers. Here's what i did:

sin(-7pi/12)

= sin(2pi/12 - 9pi/12)

= sin(pi/6 - 3pi/4)

= sin(pi/6)cos(3pi/4) - cos(pi/6)sin(3pi/4)

= (1/2)(-rad2/2) - (rad3/2)(rad2/2)

= -rad2/4 - rad6/4

= (-rad2-rad6)/4

The books says the answer is supposed to come out to be (-rad2+rad6)/4

I know i'm missing some key concept here and hours of scouring the internet has failed me. I don't know how to find the sin or cos of a negative radian measure on the unit circle; Like if i need to find the sin of -pi/4 or cos of -pi/3. I first attempted this problem using sin(-pi/4 + -pi/3) but switched to using the difference formula as to avoid using negative radians. I get a different wrong answer each way. This is my first post so i hope i have provided adequate information for you to answer my question and i hope i haven't written too much.. Thank you so very much in advance, this is driving me absolutely bonkers!!

-Nick