Question

If the common difference in an arithmetic sequence is 4, and the 20th term is 36, what is the first term?
A.
–40
B.
76
C.
40
D.
2

Answer 1

The answer is -40. I will tell you why.

Answer 2

Let a[1]=x be the first term, then
a[2]=4 + x
a[3]=8+x;
a[4]= 12 +x
a[20]=76 +x=36
x=-40

Answer 3

a[n]= 4(n-1) +a[1]
a[20]= 4(19) +x= 76 +x