Question

If −20 is the 15th term in an arithmetic sequence with a common difference of −7, what is the first term in the sequence?

Answer 1

An=A1+(n-1)d
-20=A1+(15-1)x-7
-20=A1+14 x -7
-20=A1-98
+98     +98
-98+98=0
-20+98=78
A1=78