Question

The nth term of an AP is given by (-4n+15). Find the sum of first 20 terms of this AP .

Answer 1

The nth term of an AP is
$a + (n - 1)d$
comparing this and the given
$- 4(n - 1) + 11$
Therefore first term a = 11
common difference d = -4
Sum of n term of an AP is given by
$\frac{n}{2} \times (2a + (n - 1)d)$
Hence sum of 20 terms is given by
$\frac{20}{2} \times (2 \times 11 + 19 \times - 4) \\ = - 540$
Hope this helps.

Answer 2

Answer:

Step-by-step explanation:

an = -4n + 15

a1 = -4(1) + 15 = -4 + 15 = 11

a2 = -4(2) + 15 = -8 + 15 = 7

Common difference = d = a2 - a1 = 7 - 11 = -4

S20 = 20/2 { 2*11 + 19*(-4)}

= 10 {22 + (-76) }

= 10 ( -54)

S20= -540