# How do you use a double-angle formula to find the exact value of cos2u when sin u = 7/25, where pi/2 <u < pi?

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Feb 17, 2017

$\cos 2 u = 1 - 2 {\sin}^{2} u = 1 - 2 {\left(\frac{7}{25}\right)}^{2} = 1 - \frac{98}{625} = \frac{527}{625.}$

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