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When 4 is subtracted from a number, the square of the result is 81. If the original number is positive, what is the number?
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Answer and explanation
Correct answer: 13
Let the original number be \(x\). Then \((x-4)^2=81\), so \(x-4=9\) or \(x-4=-9\). Hence, \(x=13\) or \(x=-5\). Since the original number is positive, the correct answer is 13. Exam tip: always check both possible signs when solving a squared equation.
Frequently asked questions
What is the correct answer to this question?
13
Why is this the correct answer?
Let the original number be \(x\). Then \((x-4)^2=81\), so \(x-4=9\) or \(x-4=-9\). Hence, \(x=13\) or \(x=-5\). Since the original number is positive, the correct answer is 13. Exam tip: always check both possible signs when solving a squared equation.
Which subject and chapter does this question cover?
This is a Class 10 Mathematics question. Chapter: Quadratic Equations. Topic: Word Problems and Applications.
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