+230 If the first term of an arithmetic sequence equals the third minus the second, and the fourth term is , what is the first term?
+128406 Let's see
Let the first term = a
Second term = a + d where d is the common difference
Third term = a + 2d
Fourth term = a + 3d = 8
Third term - Second term = First term
(a + 2d) - ( a + d) = a
2d - d = a
d = a
So
a + 3d = 8 so
d + 3d = 8
4d = 8
d = 2 = 1st term = a
Proof
Series is 2 , 4 , 6 , 8
6 - 4 = 2
