Question

the 7th term of an Ap is-4 and it's 13th term is -16 find it S Ap

Answer 1

A7= a+6d= -4                                 ...(i)

A13= a+12d= -16                             ...(ii)

by linear equation in two variable

from (i) and (ii)

a+6d= -4

(-)a+(-)12d= -(+)16

-6d=12

d= -2

so by substituting d value as -2 in equation (i)

we get = a+6d= -4

a+6 (-2)= -4

a=-4+12

a= 8

there by the ap is..

a=8

d=-2

so the numbers are

8,6,4,2,0,-2,-4..............