fx <- expression(x*exp(x)-exp(x));D(fx,"x")
fx <- expression(x*log(x));D(fx,"x")
fx <- expression(x*exp(x)-exp(x));D(fx,"x")
fx <- expression(x*log(x)-x);D(fx,"x")
fx <- expression(log(x)^2/2);D(fx,"x")
fx <- expression(exp(x)*(cos(x)+sin(x))/2);D(fx,"x")
f <- function(x) atan(x); integrate(f,0,1)
pi/4-log(2)/2