Question

how many terms of sequence -12,-9,-6....must be taken to make to sum 15.​

Answer 1

Step-by-step explanation:

Given series is 54,51,48,....

s

n

=

2

n

[2a+(n−1)d]

∴513=

2

n

[2×54+(n−1)−3

1026=n[108−3n+3]

1026=111n−3n

2

3n

2

−111n+1026=0

n

2

−37n+342=0

n

2

−18n−19n+342=0

n(n−18)−19(n−18)=0

(n−18)(n−19)=0

Then n=18 and n=19