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a)
$\alpha=\gamma-\beta$

$\tan\beta=\frac{10}{72}\parallel\tan^{-1}$

$\beta=7{,}907...°$
$\tan\gamma=\frac{20}{72}\parallel\tan^{-1}$
$\gamma=15{,}524...°$
$\alpha=7{,}617...°\approx7{,}6°$
b)
$\tan\alpha=\frac{x}{10{,}0}$
$\cos25{,}0°=\frac{x}{5{,}2}$
$x=5{,}2\cos25°$
$x=4{,}7128...cm$
$\tan\alpha=\frac{4{,}712...}{10{,}0}\parallel\tan^{-1}$
$\alpha=25{,}233...°\approx25{,}2°$