Explanation:
remember,
x^-n = 1/x^n
any negative exponent of 1.3 means we have to consider it as 1/1.3 with the positive exponent.
so, if the given temperature (x) is negative, 1.3 has a positive exponent and gets bigger and bigger the lower (smaller) x gets.
if the given temperature (x) is positive, 1.3 has a negative exponent and gets smaller and smaller the higher (bigger) x gets.
the domain is the interval or set of all valid x-values.
there is no need for any restriction for x.
so, the domain is all real numbers.
the question is : up to what temperature will there be heating ?
at 5°C ? yes, sure.
at 10°C ? yes, sure.
at 15°C ? some people still need heating.
at 20°C ? most likely not.
but f(20) = $0.042094266... ≈ $0.04
so, still a tiny little bit of heating is happening for the function, but in reality most likely not at all.
the information about such a limit is missing in the question. so, again no limit for x.
f(-8) means that x = -8.
f(-8) = 8(1.3)^--8 = 8(1.3)⁸ = 8×8.15730721 =
= 65.25845768
when the low outside temperature is -8°C, it would cost $65.26 to heat the building.
I don't know the answer options to the last part, but the interpretation certainly is in the context of the problem.