A few days ago
▼Brian Likes Art▼

The sine of angle theta…?

If sin of theta= – 3/5and theta is in quadrant IV, finde the exact value of cos theta.

Can you explain me on how to do this. I’m totally lost D:

Top 1 Answers
A few days ago
Truly W

Favorite Answer

Given: sin of theta= – 3/5

Given: theta is in quadrant IV

http://en.wikipedia.org/wiki/Cosine#Identities shows that :

sin(theta)^2 + cos(theta)^2 = 1

So…

(-3/5)^2 + cos(theta)^2 = 1

cos(theta)^2 = 1 – (-3/5)^2

cos(theta)^2 = 1 – 9/25

cos(theta)^2 = 16/25

cos(theta) = +/- (4/5)

Since theta is in the 4’th quadrant, and the cosine function is positive in the 4’th quadrant, the correct value of cos(theta) is +4/5

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