Question : Find the sum of the first five terms of the following series: $\frac{1}{1×4} + \frac{1}{4×7}+\frac{1}{7×10}+...$
Option 1: $\frac{9}{32}$
Option 2: $\frac{7}{16}$
Option 3: $\frac{5}{16}$
Option 4: $\frac{1}{210}$
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Correct Answer: $\frac{5}{16}$
Solution : This series is in the form of $\frac{1}{(3n-2)(3n+1)}$ where n starts from 1. The difference between the two fractions: $\frac{1}{(3n-2)(3n+1)} = \frac{1}{3} \left(\frac{1}{3n-2} - \frac{1}{3n+1}\right)$ The sum of the first five terms: $⇒S_5 = \frac{1}{3} \left[\left(1 - \frac{1}{4}\right) + \left(\frac{1}{4} - \frac{1}{7}\right) + \left(\frac{1}{7} - \frac{1}{10}\right) + \left(\frac{1}{10} - \frac{1}{13}\right) + \left(\frac{1}{13} - \frac{1}{16}\right)\right]$ $⇒S_5 = \frac{1}{3} \left( 1 - \frac{1}{16}\right) = \frac{15}{48} = \frac{5}{16}$ Hence, the correct answer is $\frac{5}{16}$.
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Question : The next number of the sequence $\frac{1}{2},\frac{3}{4},\frac{5}{8},\frac{7}{16},........$ is:
Question : If $\left(4y-\frac{4}{y}\right)=11$, find the value of $\left(y^2+\frac{1}{y^2}\right)$.
Question : What is the value of $\frac{7 \times 4 \div 8}{5 \times 25 \div 125}+\frac{5 \times 4 \div 8}{8 \times 4 \div 16}-\frac{7 \times 4 \div 2}{8 \times 7 \div 4}$?
Question : Find the value of the given expression: $\frac{(4\frac{1}{3}+3\frac{1}{3}\times 1\frac{4}{5}\div 3\frac{3}{4}\times (1\frac{1}{2}+1\frac{1}{3}))}{(\frac{2}{3}\div \frac{5}{6}\times \frac{2}{3})}$
Question : If $\left(3 y+\frac{3}{y}\right)=8$, then find the value of $\left(y^2+\frac{1}{y^2}\right)$.
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