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Math Help - turning trigonomic functions into surds

  1. #1
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    turning trigonomic functions into surds

    how do i turn cos45 into a surd? or anyother function for that matter? In part of the equation below;

    -2 x (x+2) x 4 x cos45

    How do i find x?
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    Hello,
    Quote Originally Posted by Flexible View Post
    how do i turn cos45 into a surd? or anyother function for that matter? In part of the equation below;

    -2 x (x+2) x 4 x cos45

    How do i find x?
    cos(45) is supposed to b a known value, that is sqrt(2)/2

    this can be "proved" this way :
    cos(45)=sin(45) (because cos(90-x)=sin(x))
    we know that cosē(x)+sinē(x)=1
    hence 2cosē(x)=1
    cosē(x)=1/2
    cos(x)=+ or - 1/sqrt(2)=+ or - sqrt(2)/2

    since 45 is in the first quadrant, its cosine is positive. hence it's sqrt(2)/2
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