Find the sum of the following APs:
(i) $2, 7, 12,……$ to $10$ terms.
(ii) $-37, -33, -29$ …… to $12$ terms.
(iii) $0.6, 1.7, 2.8, ……$ to $100$ terms.
(iv) $\frac{1}{15}, \frac{1}{12}, \frac{1}{10}, ……..,$ to $11$ terms.
To do:
We have to find the sum of the given APs.
Solution:
(i) $a=2, d=7-2=5, n=10$
We know that,
$S_{n}=\frac{n}{2}[2 a+(n-1) d]$
$S_{10}=\frac{10}{2}[2 \times 2+(10-1) 5]$
$=5(4+45)$
$=5 \times 49$
$=245$
(ii) $a=-37, d=-33-(37)=-33+37=4, n=12$
We know that,
$S_{n}=\frac{n}{2}[2 a+(n-1) d]$
$S_{12}=\frac{12}{2}[2 \times(-37)+(12-1) 4]$
$=6(-74+44)$
$=6 \times(-30)$
$=-180$
(iii) $a=0.6, d=1.7-0.6=1.1, n=100$
We know that,
$S_{n}=\frac{n}{2}[2 a+(n-1) d]$
$S_{100}=\frac{100}{2}[2 \times 0.6+(100-1) 1.1]$
$=50(1.2+108.9)$
$=50 \times 110.1$
$=5505$
(iv) $a=\frac{1}{15}, d=\frac{1}{12}-\frac{1}{15}=\frac{5-4}{60}=\frac{1}{60}, n=11$
We know that,
$S_{n}=\frac{n}{2}[2 a+(n-1) d]$
$S_{11}=\frac{11}{2}[2 \times \frac{1}{15}+(11-1) \frac{1}{60}]$
$=\frac{11}{2}[\frac{2}{15}+\frac{10}{60}]$
$=\frac{11}{2}(\frac{8+10}{60})$
$=\frac{11}{2} \times \frac{18}{60}$
$=\frac{33}{20}$
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