Algebra
posted by Celina .
Another question:
A cell phone company charges $60 for a cell phone and $40 per month under its economy plan. Formulate and graph a model that can be used to determine the total cost, C(t), of operating a cell phone for t months. Find the total cost for 5 and 1/2 months of service and the domain of C.
I understand everything except how you would find the domain of C. Please walk me through it so I can apply it to other problems. Thank you so much!

Algebra 
Reiny
did you have C(t) = 40t + 60 ?
now the graph would only make physical sense for the part in the first quadrant.
the yintercept is the point (0,60), (even with no calls you would pay $60)
the graph would continue to rise as t increases.
so the lowest Cost is 60 and there is really no upper limit, but we have to be reasonable about large values of t
e.g. if t = 1000, this would make no sense because 1000 months from now, cells phones may only be found in museums.
I would say the domain of t is any reasonable value of t , t > 0
you said "domain of C", what you meant is the range of C
domain deals with the horizontal variable,
range deals with the vertical variable of your function. 
Algebra 
Celina
That's funny, because in my packet, it said to "find the domain of C." But I see how that would not make sense.
Thanks!
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