Prove that: `cos(pi/4-A)cos(pi/4-B)-sin(pi/4-A)sin(pi/4-B)="sin"(A+B)`

Explanation

To prove that

\[ \cos\left(\frac{\pi}{4} - A\right) \cos\left(\frac{\pi}{4} - B\right) - \sin\left(\frac{\pi}{4} - A\right) \sin\left(\frac{\pi}{4} - B\right) = \sin(A + B) \]

we can use the cosine of sum formula.

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