Is MS Excel calculating it correctly?


* City Maths teacher finds that MS Excel software calculates ‘wrongly’ the factorials of numbers greater than 20, advises not to use the software with blind faith

Staff Reporter
The software giant Microsoft appears to have left a blind spot in the widely used product MS Excel. A city-based Mathematics teacher Hemant Ganjre has found out a problem with the software, and has advised users not to use MS Excel with blind faith while doing calculations.
Ganjre told ‘The Hitavada’ that MS Excel’s versions of 2003, 2007, 2010, and 2013 calculate ‘wrongly’ the factorials of numbers greater than 20. “This is a very big mistake and cannot be neglected as factorial or FACT function is used for higher studies,” he said. Ganjre, who also teaches Vedic Mathematics, said that he found out the problem with the software from the number of zeroes at the end of the factorials calculated by using the software and calculations done manually.
MS Excel is widely used software. It has many functions like SUM, MAX, MIN, FACT (factorial) etc. The FACT function returns the factorial of any number. In higher studies like B Sc and engineering, aeronautics, satelllite industries, etc factorial has great importance. Factorial of a number is the product of multiplication of a series of descending natural numbers from that number till 1. For instance, factorial of 5 will be the product of 5x4x3x2x1, that is, 120. Similarly, the factorial of 10 (denoted as (10!)) will be the product of 10x9x8x7x6x5x4x3x2x1, that is, 36,28,800.
According to Ganjre, MS Excel shows the factorials up to 20 correctly. After 20, the factorials of further numbers are calculated ‘wrongly’. For instance, the factorial of 21 is shown as 51090942171709400000 in MS Excel. As per the calculations done by Ganjre, the factorial of 21 is 51090942171709440000. Similarly, factorials of 22, 23, 24, 25 also are calculated ‘wrongly’ in MS Excel while manual calculations show exact results. “Even if we do not use FACT function, there is mistake in the simple multiplication to multiply all the numbers in factorial too,” Ganjre said.
There are several websites for calculation of factorials. These websites also differ in results produced by FACT function in MS Excel and their calculations using simple multiplication. Ganjre said that he was trying to get in touch with Microsoft to inform it about the problem with FACT function in MS Excel. “For now, I just appeal to the people using MS Excel not to have blind faith in factorial calculations,” he added. 

(Published in 'The Hitavada' CityLine on November 17, 2014) 

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