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What is the product of ( (r+9)(r-9) )?
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Answer and explanation
Correct answer: \(r^2-81\)
This uses the identity \((a+b)(a-b)=a^2-b^2\). Taking \(a=r\) and \(b=9\), \((r+9)(r-9)=r^2-9^2=r^2-81\). \(r^2+81\) is incorrect because conjugate binomials give a difference of squares. Exam tip: the middle terms cancel in this identity.
Frequently asked questions
What is the correct answer to this question?
\(r^2-81\)
Why is this the correct answer?
This uses the identity \((a+b)(a-b)=a^2-b^2\). Taking \(a=r\) and \(b=9\), \((r+9)(r-9)=r^2-9^2=r^2-81\). \(r^2+81\) is incorrect because conjugate binomials give a difference of squares. Exam tip: the middle terms cancel in this identity.
Which subject and chapter does this question cover?
This is a Class 9 Mathematics question. Chapter: Exploring Algebraic Identities. Topic: Algebraic identities.
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