One is to just calculate the sin of each - you might need to use the formulas for half angle (15 degrees is half of 30, sin30= .5)

Another is to note that the sin 75 = cos 15 (since 15 = 90-75), So sin15 + sin 75 = sin 15 + cos 15. You might then need to use the trig values of 30 degrees, and the equations for the sin and cos of half-angles (where 15 degrees of course is half the 30-degree angle).

A list of trig functions are here:

http://www.sosmath.com/trig/Trig5/trig5/trig5.html

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