# Question 9 and 10, Exercise 4.7

Solutions of Question 9 and 10 of Exercise 4.7 of Unit 04: Sequence and Series. This is unit of Model Textbook of Mathematics for Class XI published by National Book Foundation (NBF) as Federal Textbook Board, Islamabad, Pakistan.

## Question 9

Rewrite the sum using sigma notation: $\frac{1}{2}+\frac{2}{3}+\frac{3}{4}+\frac{4}{5}+\frac{5}{6}+\dots$

** Solution. **

$$ \frac{1}{2} + \frac{2}{3} + \frac{3}{4} + \frac{4}{5} + \frac{5}{6} +... = \sum_{k=1}^{\infty}\frac{k}{k+1} $$

## Question 10

Rewrite the sum using sigma notation: $3+6+9+12+15$

** Solution. **

$$3+6+9+12+15=\sum_{k=1}^{5}3k$$

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