Sinx = 1
x = π/2 + 2πk
0 ≤ π/2 + 2πk ≤ 4π
-π/2 ≤ 2πk ≤ 7π/2
-1/4 ≤ k ≤ 7/4
k = 0; 1
x = π/2
x = π/2 + 2π = 5π/2
-2; -4; -8;...
q = -4:(-2) = 2
S₇ = b₁(q⁷-1)/(q -1)
S₇ = -2*(2⁷-1)/(2 -1) = -2*127 = -254
a) 1/9-a²= (1/3)²-a²=(1/3-a)(1/3+a)
42:2 1/3=42:7/3=42*3/7=42/1*3/7=18