Have you ever before owned one of those Magic 8 Balls? lock look choose comically oversized pool balls, however have a flat window built right into them, so that you can see what"s insidea 20-sided die floating in poisonous opaque blue goo. Supposedly, the billiard ball has prognostic powers; every you need to do is ask that a question, provide it a shake, and slowly, mystically, favor a petroleum-covered seal arising from one oil spill, the dice will climb to the small window and also reveal the price to your question.

You are watching: How does the discriminant determine the number of solutions

The quadratic equation consists of a Magic 8 round of sorts. The expression b2 - 4ac from in ~ the radical sign is dubbed the discriminant, and it deserve to actually determine for girlfriend how many solutions a given quadratic equation has, if you don"t feel choose actually calculating them. Since an unfactorable quadratic equation needs a the majority of work to fix (tons of arithmetic abounds in the quadratic formula, and also a totality bunch of measures are required in the perfect the square method), it"s often advantageous to gaze into the mystic beyond to make certain the equation also has any real number solutions before you spend any kind of time in reality trying to find them.


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The discriminant is the expression b2 - 4ac, i beg your pardon is defined for any type of quadratic equation ax2 + bx + c = 0. Based upon the sign of the expression, you have the right to determine how countless real number options the quadratic equation has.


Here"s how the discriminant works. Provided a quadratic equation ax2 + bx + c = 0, plug the coefficients right into the expression b2 - 4ac to check out what results:

If you obtain a confident number, the quadratic will have two distinct solutions.If you acquire 0, the quadratic will certainly have precisely one solution, a dual root.

The discriminant isn"t magic. It simply shows how necessary that radical is in the quadratic formula. If that radicand is 0, because that example, then you"ll get

a single solution. If, however, b2 - 4ac is negative, climate you"ll have actually a negative inside a square root authorize in the quadratic formula, an interpretation only imaginary solutions.

Example 4: there is no calculating them, identify how countless real solutions the equation 3x2 - 2x = -1 has.

Solution: set the quadratic equation equal to 0 by including 1 come both sides.

3x2 - 2x + 1= 0
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Problem 4: there is no calculating them, determine how countless real solutions the equation 25x2 - 40x + 16 = 0 has.


Set a = 3, b = -2, and also c = 1, and also evaluate the discriminant.

b2 - 4ac=(-2)2 - 4(3)(1)= 4 - 12= -8

Because the discriminant is negative, the quadratic equation has no actual number solutions, only two imagine ones.

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Excerpted native The complete Idiot"s overview to Algebra 2004 by W. Michael Kelley. All civil liberties reserved including the ideal of reproduction in totality or in component in any type of form. Offered by plan with Alpha Books, a member that Penguin group (USA) Inc.