Question

Find the value of cos2a,when:
SinA=4/5

Answer 1

Answer:

-7/25

Step-by-step explanation:

cos(2a) =cos(a+a)

= (cos a)^2 - (sin a)^2

= (3/5)^2 - (4/5)^2

= 9/25 - 16/25 = -7/25

Note that when sinA = 4/5 , it means A = 37°

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