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7) Cos A = (AB² + AC² - BC²) / (2 . AB . AC)
cos A = (5² + 6² - 7²) / (2. 5 . 6)
cos A = (25 + 36 - 49) / 60
cos A = 12 / 60 = 1/5 = sa/mi
de = √(5² - 1²) = √24 = 2√6
sin A = de/mi = 2√6 / 5
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