Which of the following functions are linear?
f(x)=2x+1f(x) = 2x + 1f(x)=2x+1
f(x)=x2f(x) = x^2f(x)=x2
f(x)=5f(x) = 5f(x)=5
f(x)=1xf(x) = \frac{1}{x}f(x)=x1