1.

Write the principal value of cos–1(cos 680°).

Answer»

Given cos-1 (cos 680°)

= cos-1 (cos (720° - 40°))

= cos-1 (cos (2 × 360° - 40°))

= cos-1(cos 40°)

= 40°

∴ cos-1 (cos 680°) = 40°



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