In an arithmetic progression, first term a=13 and common difference d=-4. Find the sum of the first 9 terms.
Note
For an arithmetic progression, do not add all the terms one by one; use the AP sum formula. First identify the first term a, the common difference d, and the number of terms n. The sum is S_n=n/2[2a+(n-1)d]. Thus S_9=9/2[2(13)+(9-1)(-4)]=-27. So the required sum is obtained directly from the formula after substitution.