ShmoopTube

00:04

Using the Discriminant, a la Shmoop.

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It's never okay to discriminate… [Kids fighting on a bus]

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…but you can definitely use the discriminant to help answer some of life's most difficult [Magic discriminant ball]

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questions.

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In the quadratic formula b2 – 4ac is the discriminant. [Teacher writing on a whiteboard]

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The discriminant determines the nature and number of solutions of a quadratic equation.

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If b2 – 4ac is positive and greater than zero, there will be two distinct, real solutions. [2 real solutions appear from the magic discriminant ball]

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If b2 – 4ac is zero, there will be one distinct, real solution. [1 real solution appears from the magic discriminant ball]

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But, if b2 – 4ac is negative and less than zero, there are no real solutions.

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Let's get quadratic for a couple of examples.

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Our first example is x-squared plus three-x plus five equals zero. [The equation is written out]

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Plugging these bad boys into the discriminant, b squared minus 4ac, we get three-squared

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minus four-times-one-times-five.

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This simplifies to nine minus twenty, which is negative eleven.

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This is less than zero…so there are no real solutions. [Guy holding his arms up]

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What does “no real solutions” mean? [Students in class]

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If we take a look at the graph, the parabola does not touch the x-axis at any x-value. [Finger going along the x-axis]

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So it's not real, it doesn't exist.

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For our second example, we have the equation...

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x-squared minus six-x minus ten equals zero. [The equation is written out]

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Plugging this into the discriminant it looks like...

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six-squared minus four times negative-ten times one.

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This is thirty-six plus forty, which is seventy-six.

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Seventy-six is greater than zero.

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So there are two distinct, real solutions to this problem. [Guy looks happy]

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By looking at the graph, we can see that there are two real solutions because the parabola [Arrows point to the solutions on the graph]

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touches the x-axis at two distinct x-values.

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Okay, so the Magic Discriminant Ball might not be able to help you with some of life's [Guy asking the ball a random question]

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biggest questions… [The magic discriminant ball looks angry]

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… but using the discriminant can help you ace your next math test. [Guy looks happy as he gets an A+ on his test]

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