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plot error

Anyone can help me ?
How can i fix this?There is error in line 23
% this program computes and plots the exact cylinder volume
clear();
r = 8; % compression ratio
s = 100;   % stroke (mm)
b=s;         %bore=stoke
len= 150; %connecting rod length (mm)
Vd= b^2*s*pi/4;     %displacement volume
Vc=Vd/(r-1)       %clearence volume
a=s/2;            % radius of crank shaft
theta=-180:1:180; %crankangle theta vector
y =len+a-((len^2-a^2*sind(theta).^2).^(1/2) +a*cosd(theta));        %  .^2).^  nokta istiyor  exact 
vol= Vc+y*b^2*pi/4;  % exact volume
syms theta
dvdtheta=vol
diff(dvdtheta) %dv /dtheta 
%plot results
figure;
plot(theta,diff(dvdtheta),'--','linewidth',2);
xlabel('Crank Angle (deg)','fontsize', 18);
ylabel('dv/dtheta mm3','fontsize', 18);
axis ([-200 200 -1000000     1000000])
legend( 'dv/dtheta','Location', 'North');

Best Answer

See the difference between following code and your code
% this program computes and plots the exact cylinder volume
clear();
r = 8; % compression ratio
s = 100;   % stroke (mm)
b=s;         %bore=stoke
len= 150; %connecting rod length (mm)
Vd= b^2*s*pi/4;     %displacement volume
Vc=Vd/(r-1);       %clearence volume
a=s/2;            % radius of crank shaft
theta=-180:1:180; %crankangle theta vector
y =len+a-((len^2-a^2*sind(theta).^2).^(1/2) +a*cosd(theta)); %  .^2).^  nokta istiyor  exact 
vol= Vc+y*b^2*pi/4;  % exact volume
% syms theta % no need for symbolic variable
dvdtheta=vol;
diff(dvdtheta) %dv /dtheta 
%plot results
figure;
plot(theta,gradient(dvdtheta),'--','linewidth',2); % use gradient function
xlabel('Crank Angle (deg)','fontsize', 18);
ylabel('dv/dtheta mm3','fontsize', 18);
% axis ([-200 200 -1000000 1000000])
legend( 'dv/dtheta','Location', 'North');
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