(adsbygoogle = window.adsbygoogle || []).push({}); 1.x = rcos(a)cos(a)

x is known

r is known

I cant seem to get this!

x = -2

r = 7

therefore

-2/7 = cos(a)*cos(a)

so..

-2/7 = cos^2(a)

i know cos^2(a) = 1 + cos2a / 2

-2/7 = 1 + cos2a / 2

so

-4/7 -1 = cos2a

I cannot take cos inverse of this value since it is "greater" than 1 (-1.57)

the book says the angle is 106.6 degrees!!!

HELP!

2. Relevant equations

3. The attempt at a solution

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# Solving an Equation with Cosine

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