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The tangent represents the sides making up the right angle in a right angle triangle. To find out the answer to what you have asked, the simplest way is to assume that you are talking about a right angle triangle.
Step One
Find the hypotenuse. Use a variation of the Pythagorean Theorem.
opposite^2 + adjacent^2 = hypotenuse^2
5^2 + 12^2 = x^2
25 + 144 = x^2
x^2 = 169
sqrt(x^2) = sqrt(169)
x = 13
Step Two
Find the sine.
When the tangent is given, the opposite in this case is 5.
That's because the opposite is always the numerator of a tangent.
Sin(theta) = 5/13 Answer
