Question

find the sum of the first 15 multiples of 8
​

Answer 1

Answer:

120

=15×8=120

=8×15=120

so the answer is 120

Answer 2

$\tt \blue{Solution:-}$

$\: \: \: \: \: \: \: \: \: \: \: \: \:$

$\footnotesize\tt{Multiples \: of \: 8 \: are \: 8,16,24,32,40....}$

$\: \: \: \: \: \: \: \: \: \: \: \: \:$

$\footnotesize\tt \blue{These \: are \: in \: Arithmetic \: Progression \: (A.P)}$

$\: \: \: \: \: \: \: \: \: \: \: \: \:$

$\footnotesize\tt {First \: term = 8}$

$\footnotesize\tt {Common \: difference (d)= 8}$

$\footnotesize\tt{Number \: of \: terms (n)= 15}$

$\: \: \: \: \: \: \: \: \: \: \: \: \:$

$\footnotesize\tt { {s}^{n} = \frac{n}{2}[2a + (n - 1)d ]}$

$\footnotesize\tt { {s}^{15} = \frac{15}{2}[2 \times 8 + (15 - 1)8 ] }$

$\footnotesize\tt { {s}^{15} = \frac{15}{2}[16+ (14)8] }$

$\footnotesize\tt { {s}^{15} = \frac{15}{2}[128] }$

$\footnotesize\tt { {s}^{15} = \frac{15 \times 128}{2} }$

$\footnotesize\tt { {s}^{15} = \frac{15 \times \cancel{128}}{ \cancel{2}} }$

$\footnotesize\tt { {s}^{15} = {15 \times \: 64 }}$

$\boxed{ \underline{ \underline{\footnotesize\tt { {s}^{15} = {960}}}}}$

$\: \: \: \: \: \: \: \: \: \: \: \: \:$

$\footnotesize\tt \blue{Sum \: of \: first \: 15 \: multiples \: of \: 8 \: is \: 960}$