0 + 3/13 + ( - 9/13) =
= 3/13 - 9/13 = - 6/13
(x^2+2xy+y^2)-4=(x+y)^2-2^2=(x+y+2)(x+y-2).
см вложение...............................................
1/((tga+ctga)*sin^2a) = 1/((sin^2a+cos^2a)*tga) = 1/tga
cos(pi/7)*cos(4pi/7)*cos(5pi/7) = 2*sin(pi/7)*cos(pi/7)*cos(4pi/7)*cos(5pi/7)/(2*sin(pi/7)) = sin(2pi/7)*cos(4pi/7)*cos(5pi/7)/(2*sin(pi/7)) = 2*cos(2pi/7)*sin(2pi/7)*cos(4pi/7)*cos(5pi/7)/(4*sin(pi/7)*cos(2pi/7)) = sin(4pi/7)*cos(4pi/7)*cos(5pi/7)/(4*sin(pi/7)*cos(2pi/7)) = sin(8pi/7)*cos(5pi/7)/(8*sin(pi/7)*cos(2pi/7)) = -sin(pi/7)*cos(pi-(2pi/7))/(8*sin(pi/7)*cos(2pi/7)) = 1/8
Решение:
Согласно формулы: S_n=a1+d*(n-1)
S_45=-50+1,2*49=8,8
Ответ 8,8