Question

What is the sum of first ten terms of A.P. 15,10,5,...?

Answer 1

a= 15
d= -5
therfore for 10th term
an= a+9d
an= 15 + 9(-5)
= 15 -45
= -30
sn = 1/2 (a +an)
= 1/2 (15-30)
= 1/2*-15
= -7.5 ans...