Question

find the sum of first 18 terms of an ap 9,7,5,3

Answer 1

here
a=9
d=7-9
=-2
S18=n/2(2a+(n-1)d
=18/2(2×9+(18-1)×-2
=9(18+(-34)
=9×(-16)
=-144

Answer 2

a=9,d= 2
Sn= n/2(2a+(n-1)d)
S18= 18/2 (2*9+(18-1)2)
=9(18+17*2)
=9(18+34)
=9*52
=468