A(3;9),B(0;9),C(4;2)
AB(0-3;9-9)=(-3;0) ; AB=|-3|=3
BC(4-0;2-9)=(4;-7) ; BC=√4^2+(-7)^2=√65
CA(3-4;9-2)=(-1;7) ; CA=√7^2+(-1)^2=5√2
по теореме косинусов
cosA= AB^2+CA^2- BC^2 / 2*AB*CA = 3^2+(5√2)^2-(√65)^2 / 2*3*5√2= -√2/10
cosB= AB^2+BC^2- CA^2 / 2*AB*BC = 3^2+(√65)^2-(5√2)^2 / 2*3*√65 =4/√65=4√65/65
cosC= CA^2+BC^2- AB^2 / 2*CA*BC =
= (5√2)^2+(√65)^2-3^2 / 2*5√2*√65 =53/5√150=53√150/750