Question Number 8582 by thomas last updated on 16/Oct/16
$$\mathrm{9}×\mathrm{9} \\ $$
Answered by Rasheed Soomro last updated on 17/Oct/16
$$\mathrm{9}×\mathrm{9} \\ $$$$=\left(\mathrm{10}−\mathrm{1}\right)\left(\mathrm{10}−\mathrm{1}\right) \\ $$$$=\left(\mathrm{10}−\mathrm{1}\right)^{\mathrm{2}} \\ $$$$=\left(\mathrm{10}\right)^{\mathrm{2}} −\mathrm{2}\left(\mathrm{10}\right)\left(\mathrm{1}\right)+\left(\mathrm{1}\right)^{\mathrm{2}} \\ $$$$=\mathrm{100}−\mathrm{20}+\mathrm{1} \\ $$$$=\mathrm{80}+\mathrm{1} \\ $$$$=\mathrm{81} \\ $$