-26cos(3п/2 -α ) =-26*(-cos α) = 26 * cos α
26 * (-5/13) = -10
1) a+a+d+a+2d=24;
3a+3d=24;
a+d=8;
a=8-d.
2) a²+(a+d)²+(a+2d)²=210;
a²+d²+16d-82=0;
(8-d)²+d²+16d-82=0;
d²=9;
d=3 или d=-3;
a1=8-3=5, a2=8, a3=11;
или
a1=8+3=11, a2=8, a3=5.
Ответ: 5;8;11 или 11;8;5.
x² - 10x + 9 = x² - 2 * x * 5 + 5² - 5² + 9 = (x² - 10x + 25) - 25 + 9 =
= (x - 5)² - 16
(x - 5)² - 16 = (x - 5)² - 4² = (x - 5 - 4)(x - 5 + 4) = (x - 9)(x - 1)