13. COSX= 1.5 under the radical number, or 1.5 under the radical number, because 1.5 under the radical number is greater than 1.5 under the radical number and less than 1, the original formula is not valid.

Because SINX= -π/4 under the cube root and its absolute value is less than 1, it is possible.

There are too many answers at the back of the general workbook. Find it yourself.