Question

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

Answer 1

Hint:

sin(2A) = 2*sin(A)*cos(A)

So you need to find the sin(A) first using the value of cos(A)

Answer 2

hmm how would i do that
?

Answer 3

Using the idea

cos^2 + sin^2 = 1

Answer 4

so .6

Answer 5

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

Answer 6

so sin(A) = -0.6

Answer 7

now use

sin(2A) = 2*sin(A)*cos(A)

Answer 8

-.96?

Answer 9

yep