Dana F. answered 05/21/16
Tutor
5.0
(538)
Certified math teacher with 35+ plus years of teaching experience
use -2 as the first number and -14 as the fifth number because we do not know what the first number is at this point
an=a1 +(n-1)d
-14 = -2 +(5-1)d
-14= -2 +4d
-14+2= 4d
-12= 4d
d= -3
so now we can use -2 as the twelfth number and use it to find the first number
a12= a1 + (n-1)d
-2 = a1 + (12-1) -3
-2= a1 + (11)-3
-2 +33 = a1
31 =a1
so the first eight terms are 31,28,25,22,19,16,13,10 and their sum is 164
Linda C.
Memphis, TX