Find all the asymptotes of the following functions a fxe1x b

Find all the asymptotes of the following functions:
a) f(x)=e^(-1/x)
b) f(x) =(x-1)/(4x-x^3)
c) f(x) =(1-(3/x))^x

a) 1 = 1 b) diff (f) = 1/(4*x - x^3) + ((3*x^2 - 4)*(x - 1))/(4*x - x^3)^2 at infinity, diff f(x) = 0 and f(x) = 0 Hence, y= 0 is the asymptote c) diff (f) = (3*(1 - 3/x)^(x - 1))/x + log(1 - 3/x)*(1 - 3/x)^x at infinity, diff f(x) = 0 and f(x) = exp (-3) Hence asymptote is y = exp (-3)