what is the value of ( cos²67° - sin²23°) ?
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cos^2 67° - sin^2 23° = cos^2 67° - {sin 23°}^2
= cos^2 67° - {sin(90 - 67)°}^2
= cos^2 67 - {sin(90 - 67)°}^2 ..... [sin(90 - Ф) = cos Ф]
= cos^2 67° - cos^2 67°
= 0
cos^2 67° - sin^2 23° = 0
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