kenneyfamily
if cos A= 0.8 and 270 < A < 360, find the exact value of sin2A
a)0.28
b)0.96
c)0.96
d)0.6



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jim_thompson5910
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Hint:
sin(2A) = 2*sin(A)*cos(A)
So you need to find the sin(A) first using the value of cos(A)

kenneyfamily
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hmm how would i do that
?

jim_thompson5910
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Using the idea
cos^2 + sin^2 = 1

kenneyfamily
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so .6

jim_thompson5910
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but it's negative because we're in quadrant IV (sine is negative here)

jim_thompson5910
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so sin(A) = 0.6

jim_thompson5910
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now use
sin(2A) = 2*sin(A)*cos(A)

kenneyfamily
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.96?
