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How do you use double angle formula to determine cos 120?

1 Answer

Jun 5, 2015

Double Angle formula for Cosine (one version):
$XXXX$ $color(white)("XXXX")$ $cos (2 x) = 2 {cos}^{2} (x) - 1$ $cos(2x) = 2cos^2(x)-1$

$cos (120^{\circ})$ $cos(120^@)$
$XXXX$ $color(white)("XXXX")$ $= cos (2 \cdot 60^{\circ})$ $=cos(2*60^@)$

$XXXX$ $color(white)("XXXX")$ $= 2 \cdot {cos}^{2} (60^{\circ}) - 1$ $= 2* cos^2(60^@) -1$

$XXXX$ $color(white)("XXXX")$ $= 2 \cdot {(\frac{1}{2})}^{2} - 1$ $= 2 * (1/2)^2 -1$

$XXXX$ $color(white)("XXXX")$ $= - \frac{1}{2}$ $= -1/2$

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