Cosa+cosb=2cos(a+b)/2cos(a-b)/2
cos8x+cos2x=0
2cos5xcos3x=0
cos5x=0
5x=π/2+πN
x=π/10+π/5*N
cos3x=0
3x=π/2+πN
x=π/6+π/3*N
X(3x+5)=0
x=0
3x+5=0
3x=-5
x=-5/3
ответы: 0 -5/3
(х² - 10х + 21)/(2х² - 15х + 7) = ((х - 3)(х - 7)) /((2х - 1)(х - 7)) = (х - 3)/(2х - 1)