\(x+\frac{13}{12}+\frac{21}{20}+...+\frac{91}{90}\)
\(=x+1+\frac{1}{12}+1+\frac{1}{20}+...+1+\frac{1}{90}\)
\(=x+\left(1+1+1+1+1+1+1\right)+\left(\frac{1}{12}+\frac{1}{20}+...+\frac{1}{90}\right)\)
\(=x+7+\left(\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{9.10}\right)\)
\(=x+7+\left(\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{9}-\frac{1}{10}\right)\)
\(=x+7+\left(\frac{1}{3}-\frac{1}{10}\right)\)
\(=x+7+\frac{7}{30}\)
\(=x+7\frac{7}{30}\)