Number theory

9 * 7 = 63 , 6 + 3 = 9 * 1

99 * 7 = 693 , 6 + 9 + 3 = 9 * 2

9 * 77 = 693 , 6 + 9 + 3 = 9 * 2

99999999 * 7 = x ,sum of digits in x = 9 * 8 =72

9 * 77777777 = x ,sum of digits in x = 9 * 8 =72

This is the easy way to get the sum of digits in product of numbers having 7 and 9 by multiplying 9 with greatest no. of 9s or 7s in the left hand side. Try it.

#NumberTheory #Math

Easy Math Editor

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Math	Appears as
Remember to wrap math in `$` ... `$` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$

Comments

...how about other numbers? what techniques you know on how to find the sum of the digits of bigger number?

Marie Mae Siarot - 7 years, 7 months ago

Nice!!!

Tanya Gupta - 7 years, 3 months ago

Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$