Find the exact value of the trig function below.

cos2Ø if sinØ = 2/5

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  1. from sinØ = 2/5, sketch a right-angled triangle
    Use Pythagoras to find cos Ø
    cos^2 Ø + sin^2 Ø = 1
    cos^2 Ø = 21/25

    cos 2Ø = cos^2 Ø - sin^2 Ø, you should know that identity
    cos 2Ø = 21/25 - 4/25 = 17/25

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  2. cos(2x) = 1 - 2sin^2(x)

    now just use your numbers.

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