Question

Consider the arithmetic sequence 15, 13.2, 12, ....
(a) Find the nth term of the sequence.
>>>13.5+1.5n？？<<<
(b) Find the number of +ve terms.
(c) What is the sum of all the +ve terms of the sequence?

@ganeshie8 @hartnn @hba

Answer 1

is that 15,13.5,12 .. ?

Answer 2

yes

Answer 3

how about b?

Answer 4

n'th term needs to be positive, >0

13.5+1.5n >0

solve this for n

Answer 5

or solve for
13.5 +1.5n = 0
to get which term =0
and then, one number less will be number of positive terms

Answer 6

1.5n>-13.5

-ve number??

Answer 7

oops, 13.5 + 1.5 n <= 0
to get which term will be non positive.

then one less than n will give you number of positive terms

Answer 8

wait for me, i have dinner first

Answer 9

i am just trying to find which terms starts to become negative
for that i equate n'th term formula with <=0

that will be first negative term

Answer 10

okk..

Answer 11

oh and your n'th term will have negative sign
because d = -1.5 (not +1.5)

15+(n-1) (-1.5) = 15 +1.5 -1.5n = 16.5 -1.5 n

Answer 12

now do

16.5-1.5n >0 to get number of positive terms

Answer 13

you would know why d=-1.5, right ?

Answer 14

yes, since it is decreasing
n>9