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Verified answer
Syarat kurva cekung ke bawah: f"(x) < 0f(x) = cos 2x
f'(x) = -2sin2x
f"(x) = -4cos 2x
maka
-4 cos 2x < 0
cos 2x > 0
-90° < 2x < 90°
-45° < x < 45°