Доказательство:
cos84° + cos36° - cos24° =
2сos((84°+36°)/2) • сos((84°-36°)/2) - cos24° = 2сos(120°/2) • сos(48°/2) - cos24° = 2сos60°• сos24° - cos24° = 2•1/2• сos24° - cos24° = сos24° - cos24° = 0.
0 = 0, равенство доказано.
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Доказательство:
cos84° + cos36° - cos24° =
2сos((84°+36°)/2) • сos((84°-36°)/2) - cos24° = 2сos(120°/2) • сos(48°/2) - cos24° = 2сos60°• сos24° - cos24° = 2•1/2• сos24° - cos24° = сos24° - cos24° = 0.
0 = 0, равенство доказано.