# Find the exact value. Find cos 2 theta y=20 r=29 x=21

Find the exact value.
Find $\mathrm{cos}2\theta$
y=20 r=29 x=21
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Tamoni5e
$\theta =\mathrm{cos}-1\left(21/29\right)$
Trig identity:
$\mathrm{cos}2\theta =2\mathrm{cos}2\left(\theta \right)-1$
$2{\mathrm{cos}}^{2}\left({\mathrm{cos}}^{-1}\left(21/29\right)\right)-1=2\left(21/29{\right)}^{2}-1=41/841$