Question

find the sum of the 1st 70 terms of the arithmetic sequence 16,6,-4,-14

Answer 1

The nth term of the sequence is:
T(n)=26-10n, so
T(1)=26-10*1=16,
T(2)=26-10*2=6,
....
T(70)=26-10*70=-674

The sum of an arithmetic sequence is the average of the first and last terms multiplied by the number of terms.

Here, we have
Sum = (T(1)+T(70)*70/2

I'll let you take it from here.