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Review: Quadratic Equation
Explanation
In this question, 625 is 25 squared. That is worth
knowing, but if you didn't know that, you can see that the answer choices are
all integers, so the square root of 625 is an integer. You could then see that
the square is between 400 and 900, so the integer must be between 20 and 30,
and it ends in 5, so the only possibility is 25. Having settled that, there are
two cases: either we are dealing with a positive number or a negative number.
Case I: 
;
;
Case II: 
;
;
The correct answer is (C).
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