Answer:
Sum of the first 70 terms is 12,775
Step-by-step explanation:
For the 9th term, the formula is;
a + 8d = 50
For the 12th
a + 11d = 65
Subtract this from the equation above;
3d = 15
d = 15/3 = 5
To get a, we have a + 8(5) = 50
a = 10
The 70th term
will be
a + 69d
= 10 + 69(5)
= 355
Sum of the first 70
= n/2(a + L)
= 70/2( 10 + 355)
= 35 * 365 = 12,775
