# find $\int_{0}^{1} x^2 e^x$ using trapezoidal rule with error less than $10^{-3}$

The question is to find $$\int_{0}^{1} x^2 e^x$$ using trapezoidal rule with error less than $$10^{-3}$$

Which is another way to say :”Find h” in the first place. So we know that :

$$f(x)=x^2 e^x$$

$$f’(x)=2xe^x+x^2e^x$$

$$f’’(x)=2e^x+4xe^x+x^2e^x= e^x(2+4x+x^2)$$

So : $$| f’’(x)|=| 2e^x+4xe^x+x^2e^x | \le 7e$$ So the upper bound (M)would be 7e

Using $$\frac{b-a}{12} h^2 M\le 10^3$$ I have to find h and then use trapezoidal rule to find T(h) But the problem is h would be really small and using trapezoidal rule would be too long to do without using mathematical programs

Any help ?

You were on the right way: just use the error formula to get

$$h^2 \leq \sqrt{\frac{12 \text{Tol}}{7e}}$$

which yields for $$\text{Tol} = 10^{-3}$$:

$$h \approx 0.025$$ and hence $$n \approx 40$$. All in all, just set $$n>40$$ and you'll observe an absolute error less that $$\text{Tol}$$ in your simulation.

The following C++ snippet shows you this fact

#include <iostream>
#include <cmath>
#include <iomanip>      // std::setprecision

double trapz(double f(double),int n, int a, int b){

double sum{f(a) + f(b)};
double h = double(b-a)/double(n);
for (int i=1; i<n; ++i) {
sum+=2*f(a+i*h);
//        std::cout << a+i*h <<std::endl; bound checking
}
return (0.5*h)*sum;
}

double f(double x){

return x*x*exp(x);
}

int main(){
double exact{exp(1)- 2};
int n;
std::cout <<"Set n: " <<std::endl;
std::cin >> n;

double res{trapz(f,n,0,1)};
std::cout << "Numerical integration with n= " << n << " yields: "<< std::setprecision(9) << res
<< " with an error of " << std::scientific <<  std::fabs(exact - res) << std::endl;

return 0;

}


If you're not familiar with C/C++, just go to your terminal (in MacOS and Linux it's the same) and type the following in order to compile your source code

g++ -o file_name -std=c++14 file_name.cpp


Then run it with

./file_name


to see the output

With $$n=40$$ I get:

Numerical integration with n= 40 yields: 0.718706544 with an error of 4.247156195e-04

• Thank you so much for your consideration . I cannot appreciate your answer enough. Dec 21 '20 at 10:51
• No worries at all. You were on the right path :-) @Negar
– VoB
Dec 21 '20 at 10:53