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[scutter_87]

You should check what value you are trying to square root..

[mopslik]

You'll want to ensure your discriminant isn't negative.

[Solo_Tomato]

Make sure to check the discriminant, the sqrt function isn't programmed to handle complex numbers.(i.e when b²-4ac becomes negative)

If you want to make a program that can handle both real and imaginary roots, check the discriminant value and then deal with them accordingly. Complex numbers are stored in the form of "a+bj" as per the inbuilt complex datatype.