Question

the common difference of an AP is -3. Find it's sum if it's first term is 75 and the last term is 33​

Answer 1

Answer:

d= -3

a=75

l=33

an=l

a+(n-1)d=an

75+(n-1)-3=33

75+-3n+3=33

78-3n=33

-3n=33-78

-3n=-45

n=-45/-3

n=15

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

=15/2(2×75+(15-1)-3)

=15/2(150-42)

=15/2×180

=680