recall that the sum of all interior angles in a triangle is 180°.
therefore, a + b + c = 180°
[tex] \bf (\stackrel{a}{50-x})+(\stackrel{b}{9x-40})+\stackrel{c}{90}=180\implies 8x+100=180
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8x=80\implies x=\cfrac{80}{8}\implies x=10\\\\
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(\stackrel{\stackrel{a}{50-x}}{50-10})=40\qquad \qquad \qquad \qquad \stackrel{\stackrel{b}{9x-40}}{9(10)-40}=50 [/tex]
