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Leave a space between the \sin and the A. Without the space, the LaTeX compiler will interpret "\sinA" as being the name of some unknown control sequence and will therefore ignore it.
On the other hand, "\csc3A" compiles correctly, because only alphabetical letters are allowed in the names of control sequences. When the compiler reaches the "3", it knows that the control sequence "\csc" had finished with the second "c", because that was followed by a number, not a letter.
In general, it is good practice to leave a space after every control sequence name. So for example [tex]\frac {\sin A + \cos^2B} {\csc 3A} [/tex] will result in.