Question #70b87

1 Answer
Y
May 13, 2017

If Quadrant I: #4/5#
If Quadrant II: #-4/5#

Explanation:

Use Pythagorean's Identity: #sin^2x+cos^2x=1#

By substitution, we get:
#(3/5)^2+cos^2a=1#

Subtract #(3/5)^2# from both sides:
#cos^2a=1-(3/5)^2#
#cos^2a=1-9/25#
#cos^2a=16/25#

Now, we take the square root of both sides:
#cosa=+-sqrt(16/25)#
#cosa=+-4/5#

Therefore, if #angleA# is in Quadrant I, #cosa=4/5#
if #angleA# is in Quadrant II, #cosa=-4/5#

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