The function returns a vector containing the value of the the variable which minimizes the formula's value, along with the minimum value itself. Note that this command looks for a local minimum. Calc basically goes downhill from the initial guess until it finds a point at which the function's value is greater both to the left and to the right. Calc does not use derivatives when minimizing a function. In general, you should use closed intervals to find literally the minimum value in that range of x , or open intervals to find the local minimum, if any, that happens to lie in that range.
Most functions are smooth and flat near their minimum values.
Thus you should set the precision to twice as many digits as you need in your answer.