cos840=cos(360+360+120)=cos120=-cos60=-0.5
4(a²-2a+1)+6a-a²-12+2a-13= 4a²-8a+4+6a-a²-12+2a-13= 3a²-21
5(a²-8a+16)-7a+a²+28-4a-18a+6= 5a²-40a+80-7a+a²+28-4a-18a+6= 6a²-69a+114
Решение во вложении
x=5+-√25-41=5+-√-16=5+-√16i^2=5+-4i