da=0.1 ; 
I=[] ; % it contains nothing 
for a = 0:da:1
N=10; 
L=1; 
dx=L/N; 
x=[ dx/2: dx :1-dx/2];
f=a*x.^2;
Integral=sum(f)*dx;
I=[ I Integral ];
end


figure
plot( [ 0:da:1 ],I )