Find max value in polyfit

Like to find the max value of this polyfit

tmp1=abs(FRF);
q1 = polyfit(f(f>0 & f <50),tmp1(f>0 & f <50),4);
z1 = polyval(q1,f);
plot(f(f>0 & f <50),z1(f>0 & f <50))

Answer 1

[Matt J]

xcrit=roots(polyder(q1)); %set derivatives to 0

xcrit(imag(xcrit)~=0)=[]; %get rid of non-real solutions

zmax=max(  polyval(q1, [0,xcrit',50])  ); %take max over critical points.

[rockstar49]

this almost worked, however recieved this error for the last line

[Torsten]

q1 as argument to polyval is missing.

[Matt J]

Yes, I fixed it.