Question

find the sum of the first 15 terms of the following ap an=7-3n

Answer 1

Answer:

-255

Step-by-step explanation:

substitute 1 and 15 in place of n in an equation

and use the sum formula substituting values from first step

Answer 2

Answer:

a =7-3=4

a2 = 7-6=1

d= -3

n=15

sn=n/2( 2a+(n-1)d)

s15 = 15/2 ( 2(4) + 14 (-3) )

= 7.5 (-34)

= -255