What is the square root of 81?

• A. 9
• B. -9
• C. 3
• D. 81

Answer: (A)

The square root finds the nonnegative number that, when multiplied by itself, gives the original number. For 81, the number that squared equals 81 is 9, since 9 × 9 = 81. There is also a negative counterpart, -9, which would also square to 81, but the square root symbol refers to the principal (nonnegative) root, which is 9. The other options don’t fit because 3 squared is 9, and 81 squared would be 6561. So the correct square root is 9.