Question

if an (-4n+15). find the sum of first 20 terms

Answer 1

$<marquee ><font face = "times new roman" size = "5">step by step explanation</font></marquee>$
${n}^{th} \: term \: is \: given \: by \: ( - 4n + 15)$
$1 {}^{st \: term } = - 4 \times 1 + 15 = 11$
Similarly,
2nd term = 7

$common \: difference \: = \\ \:7 - 11$
$= - 4$
The sum can be found by

$s = \frac{n}{2} (2a + (n - 1)d)$

$s = \\ \frac{20}{2} \times (2 \times 11 + (20 - 1) - 4)$
$s = 10 \times (22 - 76)$
$s = 10 \times - 54 \\ s = - 540$
So, the sum of the AP is -540