Find the value of cos-1(cosπ7cosπ17-sinπ7sinπ17)

Find the value of $\cos^{- 1} (\cos \frac{π}{7} \cos \frac{π}{17} - \sin \frac{π}{7} \sin \frac{π}{17})$

Matching exercise

Match the items on the right to the items on the left.

step 1

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step 4