Saturday

April 19, 2014

April 19, 2014

Posted by **stacey graham** on Friday, March 26, 2010 at 12:27am.

find cos(2theta)

- trig -
**drwls**, Friday, March 26, 2010 at 8:06amIf tan is positive and sin is negative, theta is in the third quadrant. From Pythagorean considerations, sin theta = -7/25 and cos theta = -24/25. (Imagine a 7,25,25 right triangle)

Use the double angle formula for

cos(2 theta)

= cos^2(theta) - sin^2(theta)

= (24^2-7^2)/25^2 = 527/625

2theta will be in the first quadrant, 32.5 degrees

Check: theta = 180 + 16.26 = 196.26 deg

2 theta = 392.50 => 32.5 degrees

- trig (correction) -
**drwls**, Friday, March 26, 2010 at 8:38amChange the third sentence to:

(Imagine a 7,24,25 right triangle)

**Related Questions**

Math - 1)Find cos theta if sin theta=3/5 and 90 degrees<theta<180 degrees...

trig - given that cos(theta)= (3/5) and sin (theta)<0, find sin(2 theta) cos...

math - There are no typos and no work b/c I don't know how to do them. Thanks. 1...

trig - given that 1/2pi<theta<pi and sin theta=1/5, use appropriate ...

Math - There are no typos and no work b/c I don't know how to do them. Thanks. 1...

Pre-Calculus-check answers - If cos theta = 0.8 and 270<theta<360, find ...

Algebra II - Which of the following expressions are not equal to 1? A) sin^2 ...

trig - Find sin(2theta) if tan of theta = 7/24, pi < theta < 3pi/2 ?

Trigonometry - Thank you STEVE!!!!! Suppose that Cos (theta) = 1/square root 2 ...

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