What is the value of \(20^2 + 21^2\)?
Answer and explanation
Correct answer: 841
Compute directly: \(20^2 = 400\). Use the identity for the nearby square: \(21^2 = (20+1)^2 = 20^2 + 2\times20\times1 + 1^2 = 400 + 40 + 1 = 441\). Adding gives \(400 + 441 = 841\), so the correct answer is 841.
The closest wrong choice (831) typically arises from a slip in calculating \(21^2\) or an arithmetic error when adding. Exam tip: use \((a+b)^2\) or the nearest perfect square method to avoid careless mistakes.
Frequently asked questions
What is the correct answer to this question?
841
Why is this the correct answer?
Compute directly: \(20^2 = 400\). Use the identity for the nearby square: \(21^2 = (20+1)^2 = 20^2 + 2\times20\times1 + 1^2 = 400 + 40 + 1 = 441\). Adding gives \(400 + 441 = 841\), so the correct answer is 841.
The closest wrong choice (831) typically arises from a slip in calculating \(21^2\) or an arithmetic error when adding. Exam tip: use \((a+b)^2\) or the nearest perfect square method to avoid careless mistakes.
Which subject and chapter does this question cover?
This is a Class 9 Mathematics question. Chapter: Number Systems. Topic: Exponents.
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