by Kerry Kauffman

Created on: August 01, 2009   Last Updated: August 12, 2009

The first method I use when teaching students how to calculate square root is to look to see if the number is a perfect square first. I suggest students memorize the perfect squares from 1 to 25: 1, 4, 9, 16, 25, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400, 441, 484, 529, 576, 625. So if you are asked to calculate the square root of 441, you automatically know the answer is 21, or -21, since a negative number times another negative number yields a positive number.

For non perfect squares or larger numbers which you are unsure of if the number is a perfect square, I suggest using a factor tree. For example, suppose you want to know what the square root of 48 is. It is not a perfect square since 6 times 6 equals 36 and 7 times 7 equals 49. No whole number times itself equals 48. So break it down into factors. I always suggest trying to find a perfect square as one of the factors and in this case, 16 times 3 equals 48 and 16 is a perfect square. Remember since you are dealing with square root that the factors are also square root. So the square root of 48 equals the square root of 16 times the square root of 3. Three is a prime number so you cannot break down 3 any farther using a factor tree. We know the square root of 16 is 4, so the answer is 4 times the square root of 3.

Another more difficult example, say we need to get the square root of 2025. With such a large number, most people won't know if this is a perfect square, so use the factor tree. Know that any number ending in 5 is divisible by 5. So 5 times 405 is 2025. But 405 can be broken down into factors, using the same rule, 5 times 81 is 405. So far then we have the square root of 5 times the square root of 5 times the square root of 81. Notice then that square root of 5 times the square root of 5 equals the square root of 25. Now this problem becomes simple because you notice we have 2 perfect squares here, 25 and 81. Square root of 25 is 5 and square root of 81 is 9. So the answer is 5 times 9, which is 45 and -45, since -45 times -45 equals 2025. This problem is actually a perfect square, but if you do not recognize it as such, you can use the factor tree method I just described.

When dealing with the square root of a negative number, imaginary numbers come into play. The square root of -1 equals imaginary number denoted as "i". So in the above problems if we had the square root of -48, you have square root of -1 times square root of 3 times square root of 16. The answer is 4i times the square root of 3, and -4i times square root of 3. If we have the square root of -2025, the answer is simply 45i and -45i

Hope my method helps you when trying to figure out the square root of both positive and negative numbers.

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