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r = √9² + 12² = √225 = 15
cos a = x/r = 12/15 (A)

(x,y) = (12,9) => r = √(x^2 + y^2) = √(12^2 + 9^2) = √(144 + 81) = √225 = 15
Cos a = x/r = 12/15