Trigonometric identities

With 180° < θ < 270°, we know that both cos(θ) < 0 and sin(θ) < 0.

Then from the Pythagorean identity, it follows that

sin²(θ) + cos²(θ) = 1   ==>   sin(θ) = -√(1 - cos²(θ))

and so

sin(θ) = -√(1 - (-8/13)²) = -√105/13

Recall the double angle identity for sine:

sin(2θ) = 2 sin(θ) cos(θ) = 2 (-√105/13) (-8/13) = 16√105/169

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