cosx = -2/3 and x is in quad II
then x = -2, r = 3
x^2 + y^2 = r^2
4 + y^2 = 9
y = + √5 in quad II
then sinx = √5/3
cos 2x = cos^2 x - sin^2 x
= 4/9 - 5/9 = -1/9
You are correct,
did you do it the same way?
Find the value of cos2x.
cosx = -2/3 pi/2 < x < pi
Is this -1/9?
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