Question

for an A P,a=-10,d=4.find S10​

Answer 1

a = -20

d = 4

n = 10

then S10 = ?

we know

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

=>

$\frac{10}{2} (2( - 20) + (10 - 1)4)$

=> 5( -40 + 36)

=> 5 × -4

=> -20

Answer 2

Answer:

Given a = -10 ; d = 4 n = 10

put these values in the formula

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

$s10 = \frac{10}{2} \: (2 \times - 10 + (10 - 1)4)$

= 5 (-20 + 9 ×4)

= 5 ( -20 + 36)

= 5 × 16

$= 80$

this is the answer of your question. Hope you will understand it