Question 1177648
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Hi
(0, −9) 3rd quadrant
 r = sqrt(x^2 + y^2) = sqrt(81) = ± 9
for the point for 0 ≤ 𝜃 < 2𝜋
tan θ = 0/-9 = 0  Look below:  θ is 3π/2  0r    3π/2 - π = π/2 if r < 0
two sets of polar coordinates:
(9, 3π/2)  and (-9, π/2) 
Wish You the Best in your Studies
(cos, sin)  Unit Circle:
     *[illustration wheel].
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