Dividing by x deletes a solution: the SAT math trap

Short answer: Never divide by a variable. Move everything to one side and factor.

The trap, in one question

Tempting answer: x = 9 only  (divided both sides by x)

Correct answer: x = 0 and x = 9  (x(x − 9) = 0)

Watch it in under a minute

How to spot it in 3 seconds

Never divide by a variable. Move everything to one side and factor.

FAQ

What is the dividing by x deletes a solution trap on the SAT?

Example: What are all solutions to x2 = 9x? The tempting answer is x = 9 only (divided both sides by x); the correct answer is x = 0 and x = 9 (x(x − 9) = 0).

How do you avoid the dividing by x deletes a solution trap?

Never divide by a variable. Move everything to one side and factor.

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