expression, parse(), eval()で数式を

hogeEx<-parse(text="2/3*x^3+sqrt(x)-x^4*cos(x)")
evalWithVal<-function(x,exp){
 eval(exp)
}
x<-seq(from=0,to=10,length.out=101)
plot(x,evalWithVal(x=x,exp=hogeEx),type="l")