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AC² = AC²
AB² + BC² - 2 . AB . BC cos Ф = AD² + CD² - 2 . AD . CD cos (180 - Ф)
1² + 2² - 2 . 1 . 2 . cos Ф = 4² + 3² - 2 . 4 . 3 . -cos Ф
1 + 4 - 4 cos Ф = 16 + 9 + 24 cos Ф
5 - 25 = 24 cos Ф + 4 cos Ф
-20 = 28 cos Ф
cos Ф = -20/28 = -5/7