"cosine equals adjacent over hypotenuse for a right triangle. This is not a right triangle- it does not obey the "Pythagorean theorem": \(\displaystyle 4^2+ 5^2= 16+ 25= 41\) not \(\displaystyle 7^2= 49\). Instead, use the "cosine law": in any triangle \(\displaystyle c^2= a^2+ b^2- 2ab cos(C)\) where C is the angle opposite side c.
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