Дано: ∆ABC, A (-5; -2), B (-1; 4), C (2; 2). Найти: ∠B.

CosB = (BA*BC) / (|BA|*|BC|)

BA{-5 - (-1); - 2-4}, BA{-4; -6} |BA|=√ ((-4) ² + (-6) ²), |BA|=√52

BC{2 - (-1); 2-4}, BC{3; -2}, |BC|=√ (3² + (-2) ²), |BC|=√13

cosB = (-4*3 + (-6) * (-2)) / (√52*√13)

cosB=0/26, cosB=0⇒