# Product Of Opposite Sides In A Quadrilateral

Geometry Level 5

Let $$ABCD$$ be a convex quadrilateral such that $$AB\cdot CD=12$$, $$BC\cdot AD=14$$ and $$\cos (\angle A+\angle C)=-\frac{2}{3}$$. Find the value of $$(BD\cdot AC)^2$$.

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