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tramserran
4th quadrant is positive x (cos) and negative y (sin)

Use the Pythagorean Theorem to calculate the value of y (sin).
x² + y² = c²
5² + y² = 13²
25 + y² = 169
       y² = 144
       y = 12

Since the y-value is negative in the 4th quadrant, then y = -12