posted by Jess .
If sinx= -1/4 and x terminates in the third quadrant, find the exact value of sin2x
My answer is sqrt-15/8
sin2x=2sinx cos x
= 2*-.25*cos x but
so now you have it.
check sign: if x is in the third quadrant, so 2x will be in the second quadrant, where sine is +
wait i don't understand how you did your math?
dont you use the formula sin2 theta =2sintheta cos theta?
I used that formula sin^2x+cos2x=1
-1/4 squared minus that by one and squareroot it square root of 15 over 4
you can do these a number of ways...
=sqrt(15) / 8
Awesome thanks mathmate!