Depends on the measurement of the angle. For those that are multiples of the angles of 0,30,45,60,90 degrees there are commonly accepted values that are to be memorized much like times table is. For other angles, if you need an exact value, you can look into table of values for the trigonometric functions. However, there a sort of tasks exists where a value is required to be found on your own without the use of tables and calculators. Tan of what an angle exactly do you need to find?
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