Question

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

Answer 1

a=15,d=-5
$s10 = \frac{10}{2} (2 \times 15 + 9 \times ( - 5))$
S10=5(30-45)
= -5×15
= -75

Answer 2

a=15
d=10-15
=-5
sn=n/2(2a+(n-1)d)
s10=10/2(2×15+(10-1)-5)
=5(30+(9)-5)
=5(30-45)
=5(-15)
=-75