277 Find the area of the quadrilateral $A B C D$, if $\cos \alpha=-\frac{1}{15}$.
278 Find the length of the third side of a triangle $A B C$, if $A B=4 \mathrm{~cm}, B C=10 \mathrm{~cm}, A \widehat{B C}$ is an obtuse angle and the area of $A B C$ is $12 \mathrm{~cm}^2$.
[6 $\sqrt{5} \mathrm{~cm}$ ]
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