$$\frac{1}{{20}}$$ $$ + $$ $$\frac{1}{{30}}$$ $$ + $$ $$\frac{1}{{42}}$$ $$ + $$ $$\frac{1}{{56}}$$ $$ + $$ $$\frac{1}{{72}}$$ $$ + $$ $$\frac{1}{{90}}$$ $$ + $$ $$\frac{1}{{110}}$$ $$ + $$ $$\frac{1}{{132}}$$ is equal to :
A. $$\frac{1}{8}$$
B. $$\frac{1}{7}$$
C. $$\frac{1}{6}$$
D. $$\frac{1}{10}$$
Answer: Option C
Solution(By Examveda Team)
$$\frac{1}{{20}} + \frac{1}{{30}} + \frac{1}{{42}} + \frac{1}{{56}} + \frac{1}{{72}} + \frac{1}{{90}}$$ $$ + \frac{1}{{110}}$$ $$ + \frac{1}{{132}}$$$$ = \left( {\frac{1}{{4 \times 5}}} \right) + \left( {\frac{1}{{5 \times 6}}} \right) + \left( {\frac{1}{{6 \times 7}}} \right)$$ $$ + ..... + $$ $$\left( {\frac{1}{{11 \times 12}}} \right)$$
$$ = \left( {\frac{1}{4} - \frac{1}{5}} \right) + \left( {\frac{1}{5} - \frac{1}{6}} \right) + \left( {\frac{1}{6} - \frac{1}{7}} \right)$$ $$ + ..... + $$ $$\left( {\frac{1}{{11}} - \frac{1}{{12}}} \right)$$
$$ = \frac{1}{4} - \frac{1}{5} + \frac{1}{5} - \frac{1}{6} + \frac{1}{6} - \frac{1}{7} \, + .....$$ $$ + \frac{1}{{11}}$$ $$ - \frac{1}{{12}}$$
$$\eqalign{ & = \frac{1}{4} - \frac{1}{{12}} \cr & = \frac{{3 - 1}}{{12}} \cr & = \frac{2}{{12}} \cr & = \frac{1}{6} \cr} $$
Related Questions on Number System
Three numbers are in ratio 1 : 2 : 3 and HCF is 12. The numbers are:
A. 12, 24, 36
B. 11, 22, 33
C. 12, 24, 32
D. 5, 10, 15
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