Question

Find the sum of arithmetic progressions of $20$ terms $1,5,9,13,....$. Also find last term.

Answer 1

we know,

$a_n=a_1+(n+1)d$

$\therefore$ last term $=a_{20}=a_1+(20-1)\left(4\right)$

$=1+19\times 4$

$=77$

Sum$=\frac{n}{2}$(first term+last term)

$=\frac{20}{2}(1+77)$

$=780$