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2 May, 23:15

The exact value of cos 13pi/8 is

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Answers (2)
  1. 3 May, 01:15
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    sqt root 2-sqt root 2/4
  2. 3 May, 01:37
    0
    answer is : Cos (13pi/8) = 0.3826

    Step-by-step explanation:

    We have, Cos (13pi/8)

    Since 13pi/8 can be shown as 3pi/2 < 13pi/8 < 2pi

    Hence 13pi/8 lies on fourth quadrant.

    In fourth quadrant cosine will be positive.

    Cos (13pi/8) = cos (3pi/2 + pi/8)

    applying formula cos (A+B) = cos A cosB - sinAsinB

    i. e Cos (3pi/2 + pi/8) = cos (3pi/2) cos (pi/8) - sin (3pi/2) sin (pi/8)

    ∵ Remember cos (3pi/2) = 0, sin (3pi/2) = - 1

    Cos (3pi/2 + pi/8) = 0 cos (pi/8) - (-1) sin (pi/8)

    Cos (3pi/2 + pi/8) = 0 + 0.3826

    Cos (3pi/2 + pi/8) = 0.3826

    Hence we got Cos (13pi/8) = 0.3826
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