We know:
and
These expressions have an 
term in common. We can combine them if we can find forms of the expressions in terms of :
Now, because 
(by the reflexive property), we can write:
This is our answer, A.
Hello,
(-25) - (25) = (-25) + (-25) = (-50)
Bye :)
This question is basically a ratio 1 : 42
so then it would be 2 : 84 then 3 : 126
So your answer is D 3 Gallons for 126 miles.
I guess you are asking to find the sum of the first 8 terms. If so, then:
Sum = a₁(1-rⁿ)/(1-r), where a₁ is the 1st term, r=common ratio and n=number of terms:
the 1st term a₁ =3
common ratio r = - 2 (since -6/3 = - 2, and 12/-6 = - 2, etc.)
Sum = 3[(1- (-2)⁸]/(1-2) = 3(1- 256)/(1/2)
Sum = -1530
