The discriminant is the expression inside the square root of the quadratic formula. For a quadratic ax² + bx + c = 0, the quadratic formula is x = (-b +- Sqrt(b² - 4ac))/(2a). The expression (b² - 4ac) is the discriminant. This can tell a lot about the type of roots. First, if the discriminant is a negative number, then it will have two complex roots. Because you have a real number plus sqrt(negative) and real number minus sqrt(negative).
You asked about irrational. If the discrimiant is a perfect square number {like 1, 4, 9, 16, etc.} then the quadratic will have two distinct rational roots (which are real numbers). If the discriminant is zero, then you will have a double root, which is a real rational number.
So if the discrimiant is positive, but not a perfect square, then the roots will be irrational real numbers. If the discriminant is a negative number which is not the negative of a perfect square, then imaginary portion of the complex number will be irrational.
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In the quadratic equation, b^2 - 4ac < 0.
If the determinant of the quadratic (ax² + bc + c) as worked out by b² - 4ac is a perfect square or not. If the determinant is not a perfect square then the roots are irrational.
None, if the coefficients of the quadratic are in their lowest form.
The discriminant must be a positive number which is not a perfect square.
There is no simple factorisation because the quadratic expression does not have rational roots. Irrational roots are not used in factorisation.