Here it is
function A=sect_area_cylinder(R,H,alpha)
%Cylinder of radius R and height H,
%its base-circle center coincide with the point (0,0,0).
%The intersecting plane is supposed to rotate around the
%line (x=0,y=H/2) with angle ALPHA (in degrees)
if (R<=0)||(H<=0)
error('Input positive R,H');
end
if alpha==0
%the cross-section is a circle
A=R^2*pi;
elseif alpha==90
%the cross section is a rectangle
A=2*R*H;
elseif (0<alpha)&&(alpha<90)
%the cross section is an ellipse
%of short radius R and long radius R/cos(alpha)
A=pi*R*R*cos(alpha*pi/180);
else
disp('alpha in [0,90]');
end
