Saturday

February 28, 2015

February 28, 2015

Posted by **Jess** on Saturday, August 6, 2011 at 6:14pm.

My answer is sqrt-15/8

- Precalculus -
**bobpursley**, Saturday, August 6, 2011 at 6:25pmsin2x=2sinx cos x

= 2*-.25*cos x but

sin^2x+cos^2x=1 so

cos^2x=.75

cosx=sqrt 3/4

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 +

- Precalculus -
**Jess**, Saturday, August 6, 2011 at 6:31pmwait i dont understand how you did your math?

- Precalculus -
**Jess**, Saturday, August 6, 2011 at 6:34pmdont you use the formula sin2 theta =2sintheta cos theta?

- Precalculus -
**Jess**, Saturday, August 6, 2011 at 6:36pmI used that formula sin^2x+cos2x=1

-1/4 squared minus that by one and squareroot it square root of 15 over 4

- Precalculus -
**bobpursley**, Saturday, August 6, 2011 at 6:55pmyou can do these a number of ways...

- Precalculus -
**MathMate**, Saturday, August 6, 2011 at 7:16pmI get

cos(x)

=sqrt(1-sin^2(x))

=sqrt(1-(1/4)^2)

=sqrt(15/16)

so

sin(2x)

=+2sin(x)cos(x)

=+2*(1/4)sqrt(15/16)

=sqrt(15) / 8

- Precalculus -
**Jess**, Saturday, August 6, 2011 at 7:29pmAwesome thanks mathmate!

- Precalculus :) -
**MathMate**, Saturday, August 6, 2011 at 7:42pmYou're welcome!

**Answer this Question**

**Related Questions**

Trigonometry - find sin2x, cos2x, and tan2x if sinx= -2/sqrt 5 and x terminates ...

trig - sinx = 4/5 and x terminates in Quadrant II Find sin2x and cos2x How to ...

Trig - If secx = 8 and -pi/2 < x < 0, find the exact value of sin2x Use ...

pre calc - Find the exact value of sin(x-y) if sinx=-3/5 in Quadrant III and ...

Pre Cal - Find the exact value of the cosine and the cosecant of thetha, if ...

easy trig - If sin(theta)=[sqrt(70)]/7 and theta is in Quadrant two, find the ...

simple trig - If sin(theta)=[sqrt(70)]/7 and theta is in Quadrant two, find the ...

Math - For f(x) = 2sinx + (sinx)^3 + tanx find f'(pi/3). Ok, so what I tried was...

check math - Here are my answers. Can you check if I got the right answers? ...

trig - Angle x is in the second quadrant and angle y is in the first quadrant ...