cos0+cospi/7+cos2pi/7+cos3pi/7+cos4pi/7+cos5pi/7+cos6pi/7=
=1+2сosП/2сos5П/14+2cosП/2cos3П/14+2cosП/2cosП/2cosП/14=1
X^2-3x+13=0
x=(-(-3)+-√(-3)^2-4*1*13)/(2*1)
x=(3+-√9-52)/2
x=(3+-√-43)/2
x не принадлежит реальным числам!
A) 5x^2 - 5 = 5(x^2 - 1) = 5(x+1)(x-1)
b) 10x^2 - 10y^2 = 10(x^2 - y^2) = 10(x+y)(x-y)
c) 9b^3 - b = b(9b^2 - 1) = b(3b+1)(3b-1)