Evaluate :
cos2 25° - sin2 65° - tan2 45°
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= cos2 (90° - 65°) - sin2 65° - tan2 45°
= sin2 65° - sin2 65° - tan2 45°
= - tan2 45°
= - 1
Hence, cos2 25° - sin2 65° - tan2 45° = -1.
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sin2 35° - cos2 55°
Show that :
tan 10° tan 15° tan 75° tan 80° = 1