\(\frac{1}{10}+\frac{4}{20}+\frac{9}{30}+...+\frac{81}{90}\)
\(=\frac{1}{10}+\frac{2}{10}+\frac{3}{10}+...+\frac{9}{10}\)
\(=\frac{\left(9+1\right)\left(9-1+1\right):2}{10}\)
\(=\frac{10.9:2}{10}=4,5\)
=\(\frac{1}{10}+\frac{4}{20}+\frac{9}{30}+...+\frac{81}{90}\)
= \(\frac{1}{10}+\frac{2}{10}+\frac{3}{10}+...\frac{9}{10}\)
= \(\frac{\left(9+1\right)\times\left(9-1+1\right):2}{10}\)
= \(\frac{10\times9:2}{10}\)
= 4,5
ĐS:.........................
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