That is a number known as "i" - also known as the "imaginary unit". For more information, you may want to investigate about the so-called "complex numbers". The Wikipedia article on complex numbers may be a good starting point.
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1 or -1, but 1 is generally the accepted answer.
0.5
2½ ie 2 to the power of one-half. The power "one-half" applied to any value signifies that value's square root. Do not confuse with the power "-1" which indicates the reciprocal of the given value.
Mathematicians decided that, since the square root of a negative number does not exist, they would use the first letter of "imaginary" to represent this "value".
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