function plotsurface(fn,int1,int2)
%plots a sruface defined by the function (represented by a string) fn  over intervals x in int1 and y in int2
x=int1(1)+.025*(int1(2)-int1(1))*(0:40);
y=int2(1)+.02*(int2(2)-int2(1))*(0:50);
z=[];
for i=1:41
    z=[z ;eval(strcat(fn,'(x(i),y)'))];
end
surf(x,y,z')
figure
contour(x,y,z')