If you mean x + y = 18 then it is the function of a straight line
Only if it's a function.
It is an increasing odd function.
No, f(x) = x + x^5 in not linear.
The domain of the function f (x) = square root of (x - 2) plus 4 is Domain [2, ∞)
(x + 2)(x + 4) x = -2, -4
No, but cos(-x) = cos(x), because the cosine function is an even function.
Yes, y=-2+x is a linear function.
They are at x = -3 and x = -2.
It may be a linear function in two variables.
Neither.
x(y+z)
It is x - y + 2 = 0