Matlab: Integration Approximation

I need to devise an integration method for computing the values of the function

F(x) =int(exp(-x^2)), a and b are 0 and x, respectively.

For this fixed value of x, I must demonstrate how the error in the integral changes with h using a plot.

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Here is my code:

function face = trapez(b)

%Trapezoidal rule for solving the integral of exp(-x^2) between a and b.

n=linspace(10,1,100);

a=0;

hs=(b-a)./n;

fa=exp(-0^2);

fb=exp(-b^2);

es=[]

for h=hs

int=(h/2)*(fa+fb); %Trapezoid Formula

int0=(10^(-10)/2)*(fa+fb)

e=abs(int-int0);

es=[es,e];

end

face=es;

In this code I set the "real value" of the integral of the function as having a very small h and then I plot the function. Then I made a plot of the function vs. increasing h.

n=linspace(10,1,100);

a=Trapez(3);

hs=3./n

loglog(hs,a)

The problem is the graph came out having a linear error curve, which just doesn't seem right. Should the error fluctuate like one's heart rate? Is there a problem with my code? Thanks in advance!