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Solving Quadratic Equations Using Square Roots - Problem 2

Teacher/Instructor Alissa Fong
Alissa Fong

MA, Stanford University
Teaching in the San Francisco Bay Area

Alissa is currently a teacher in the San Francisco Bay Area and Brightstorm users love her clear, concise explanations of tough concepts

Here I have a quadratic equation with no b term so right away I’m thinking I’m going to want to take the square root of both sides of an equation but first I want to get x isolated. I’m going to have to get rid of this -81 piece by adding 81 to both sides, 100x² equals 81.

Next thing I want to do is to get rid of that 100. I’m going to divide both sides by 100 so that I have x² equals 81 over 100. Okay I’m almost there in order to have regular old x and not x² I need to square root both sides. X is going to be equal to the positive and negative square root of 81 over 100.

So depending on what your textbook says you might grab a calculator and do that on your calculator or you might be able to know in your head that the square root of 81 over 100 is 9 over 10. So x is going to be equal to positive and negative 9/10.

The way I would check that is by substituting in one at a time positive 9/10 and then negative 9/10 here and making sure that’s equal to 0.

So guys these problems aren’t too bad if there’s no b term just undo everything that’s been done to x including taking the square root of both sides at the end.

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