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6 December, 06:47

Rewrite 1-cos (135) / sin (135) using half-angled idenitity

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  1. 6 December, 07:47
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    The two half angle identities we'll use are:

    (1-cos2x) / 2 = (sinx) ^2 and sin2x = 2sinxcosx

    first we'll deal with the numerator:

    1-cos (135) kind of looks like 1-cos2x, so we'll use that identity after a little rearranging:

    (1-cos2x) / 2 = (sinx) ^2

    multiply both sides by 2 and we get:

    (1-cos2x) = 2 (sinx) ^2

    Now back to our numerator:

    if 135 = 2x, then x = 67.5

    1-cos (135) = 2 (sin67.5) ^2

    So we've rewritten our numerator as 2 (sin67.5) ^2

    Now for our denominator we'll use the half angle identity:

    sin2x = 2sinxcosx

    So our denominator becomes:

    sin (135) = 2sin (67.5) cos (67.5)

    Now put it all together ...

    (2 (sin67.5) ^2) / 2sin (67.5) cos (67.5)

    The 2 on top and bottom cancel and the (sin67.5) ^2 cancels the sin67.5 on the bottom so you're left with

    sin67.5/cos67.5

    Which simplifes to

    tan67.5
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