A1=-21 n=20 d=-18+21=3
s20=(2a1+d(n-1)*n/2 =(-42
+3*19)*20/2=150
f(x) = F'(x)
F(x)=3tgx f(x) = 3/cos²x
F(x)=5ctgx f(x) = -5/sin²x
F(x)=3/cos²x = 3·(cosx)² f(x) = 3·(-2)·(cosx)⁻³·(-sinx) = 6sinx/cos³x
F(x)=-5/sin²x = -5·(sinx)⁻² f(x) = -5·(-2)·(sinx)⁻³·cosx = 10cosx/sin³x
1) 3 + 3 * (-1/3) = 3 - 1 = 2;
2) 3 + 3 * (-3) = 3 - 9 = -6;
3) 2 > -6 значит 3+3a при a = (-1/3) > 3+3a при a= -3;