Question

The Sum of the first 21 terms of the progression, -18, -15,-12........ is _____​

Answer 1

3

Explanation:

18,15,12,..........

S

n

=45=

2

n

[2a+(n−1)d]

=>

2

n

[2×18+(n−1)(−3)]=45

=>

2

n

[36−3n+3]=45

=>n(39−3n)−90=0

=>39n−3n

2

−90=0

=>3n

2

−39n+90=0

=>n

2

−13n+30=0

=>n

2

−10n−3n+30=0

=>n(n−10)−3(n−10)=0

=>(n−10)(n−3)=0

=>n=10 or n=3