10 Cos² x + 3 Cos x -1 = 0
t = Cos x, |t| ≤ 1
10t² +3t -1 = 0
D = 9 +40 = 49
t = (-3 -7)/20 = -1/2
t = (-3+7)/20 = 1/5
Cos x = -1/2
x = ± 2π/3 +2πn, n ∈ Z
Cos x = 1/5
x = ± arccos(1/5) +2πn, n ∈ Z
Sin(-240°) = -sin240° = - sin(180 + 60) = sin60° = √3/2
cos(-240°) = cos240° = cos(180 + 60) = -cos60° = 1/2
tan(-240°) = -tan240° = -tan(180+60) = -tan60° = -√3
cot(-240°) = -cot240° = -cot(180+60) = -cot60° = -1/√3
1). (x-10)*(x^2+10x+100)-x^3=x^3-1000-x^3= -1000; 2). 216-(a-6)*(a^2-6a+36)=216-a^3-216= -a^3.