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Тут - зависит от того, что вы проходили. Надо отложить от точки С вектор ВА - получим коодинаты D. Или, еще точнее, вектор ВА равен вектору СD. Формально это можно записать так (1 - 2; -2 - 4) = (x + 1; y - 5)
x + 1 = -1;
x = -2;
y - 5 = -6;
y = -1;
D (-2; -1)
угол А - это угол между векторами
АВ = (2 - 1; 4 + 2) = (1; 6); и АD = (-2-1;-1+2) = (-3; 1)
IABI = корень(1^2 + 6^2) = корень(37);
IADI = корень(1^2 + 3^2) = корень(10);
Скалярное произведение AB*AD = 1*(-3) + 6*1 = 3;
cos(A) = 3/корень(370)
