ACT Preparation / Unit 8 ACT 0716 short lesson, about 20 seconds
Values That Make an Expression Undefined
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The idea
A rational expression is undefined wherever its denominator equals zero, so those values are excluded. Find them from the original denominator, before cancelling.
In practice
(x − 1)/(x² − 4) is undefined at x = 2 and x = −2.
Where people slip
Cancelling can hide an excluded value: (x² − 9)/(x + 3) simplifies to x − 3 but is still undefined at −3.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
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