The 7th term of an arithmetic sequence is 1, and the 20th term is -25. find the 25th term.

A(7)=1
a(20)=25
a(25)=?

a(n)=a(1)+d(n-1)

a(7)=a(1) +6d=1         -11/13+6*24/13= 
a(20)=a(1)+19d=-25

a(1) +6d=1
a(1)+19d=-25 (subtract first equation from the second)

      19d-6d =-26
       13d =-26
d=-2

a(1)=1-6d=1-6*(-2)=13

a(25)= a(1)+d(25-1)= 13+(-2) *24= 13-48=-35

Are you trying to figure out the volume?