Finding Numbers for which Rational Expressions are Undefined More Options: Make a Folding Card

Storyboard Description

While taking an Algebra I test, Ryan helps Tobie cheat on it by explaining how to find numbers for which a rational expression is undefined.

Storyboard Text

• Psst, Ryan. How'd you do number 9?
• Shh, Tobie! All right, I'll tell you.
• Okay, first off, don't bother looking at the numerator, it doesn't matter here.
• Find any numbers for which the rational expression is undefined. 9.    (11x2 + 1)/(x2 - 5x - 14) x2 - 5x - 14
• Find any numbers for which the rational expression is undefined. 9.    (11x2 + 1)/(x2 - 5x - 14) x2 - 5x - 14 = 0 (x + 2)(x - 7) = 0
• Now you set the denominator equal to zero, since any fraction with a denominator of 0 is undefined. Then you factor the quadratic equation by "double-bubbling" it.
• Now you set each set of parentheses equal to 0 and solve for x in each equation. It should be -2 or 7. So those numbers are the values for x for which this rational expression is undefined.
• Find any numbers for which the rational expression is undefined. 9.    (11x2 + 1)/(x2 - 5x - 14) x2 - 5x - 14 = 0 (x + 2)(x - 7) = 0 x + 2 = 0 x = -2 or x - 7 = 0 x = 7
• AHEM!
• All right, thanks Ry-
• "Next" time?!
• Hey!
• Uh, Tobie, dude, next time you want to try this, you mind whispering a little quieter?
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