void trapezio_parallel(double a, double b, int n, double* global_result){
	double h,x,my_result,local_a, local_b;
	int i, local_n;
	int my_rank = omp_get_thread_num();
	int thread_count = omp_get_num_threads();
	h=(b-a)/n;
	local_n=n/thread_count;
	local_a=a+my_rank*local_n*h;
	local_b=local_a+local_n*h;
	my_result = (f(local_a)+f(local_b))/2.0;
	for (i=1;i<=local_n-1;i++){
		x = local_a+i*h;
		my_result += f(x);
	}
	my_result *=h;

	#pragma omp_critical
	*global_result += my_result;
}