Question

Find the sum of the first 12 terms of the sequence. Show all work for full credit.

1, -4, -9, -14, . . .

Answer 1

this is an AP series with 1st term=a=1 and common difference=d=-5
also here n=12
so S_n=(n/2)(2*a+(n-1)*d)=(12/2)(2*1+(12-1)*(-5))=-318

Answer 2

@lxoser check the solution and do tell me if you have any problem

Answer 3

Well the difference is -5, so your 12 digits would be:
1, -4, -9, -14, -19, -24, -29, -34, -39, -44, -49, -54
Add them together and you get: -318.
However @jango_IN_DTOWN is right.

Answer 4

@Ghostgate, you method is applicable only when the number of terms is less, or else it will take more time.:)

Answer 5

@jango_IN_DTOWN That is true, I remember learning this, however I forgot the equation.