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= (cos²a-sin²a)/cos²a / (cos²a + sin²a) / cos²a
= (cos²a-sin²a)/cos²a x cos²a /(cos²a + sin²a)
= (cos²a-sin²a) / (cos²a + sin²a)
= cos 2a /1
= cos 2a
ket :
sin²a+cos²a = 1
cos²a-sin²a = cos 2a