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Verified answer
Kelas 11 MatematikaBab Trigonometri Lanjut
cos 2x = 2 cos² x - 1
cos 2x = 2 . (1/√5)² - 1
cos 2x = 2 . 1/5 - 5/5
cos 2x = 2/5 - 5/5
cos 2x = -3/5