Question

IF cos x=-4/5 and π<x< 3π / 2, then cos x/2= ?
a)3/√10
b)-1/√10
c)1/√10
8)-3/√10
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Answer 1

Answer:

c) 1/√10

Step-by-step explanation:

Given, cos x=-4/5

=> 2cos²(x/2) - 1 = -4/5

=> 2cos²(x/2) = 1 - 4/5

=> 2cos²(x/2) = 1/5

=> cos²(x/2) = 1/10

=> cos(x/2) = 1 / √10

Answer 2

Step-by-step explanation:

cos x = cos 2×x/2 =( 2×cos^2 x/2) -1

4/5 +1=2 cos^2 x/2

9/10 = cos^2 x/2

so cos x/2 =3/√10

ans is (a)