Why the result of this code in C of 55?

I suggest adding the following line inside the for:

printf("i vale %d Soma vale%d\n",i, soma);

Getting:

#include <stdio.h>
#include <stdlib.h>

int main(){
    int i, soma = 0;
    for(i=1;i<=10;i++) {
        soma = soma + i;
        printf("i vale %d Soma vale %d\n",i, soma);
    }
    printf("Soma %d\n", soma);
    system("pause");
    return 0;
}

Then you will know that ...

i vale 1 Soma vale 1  //1ª Iteração
i vale 2 Soma vale 3  //2ª Iteração
i vale 3 Soma vale 6  //3ª Iteração
i vale 4 Soma vale 10  //4ª Iteração
i vale 5 Soma vale 15  //5ª Iteração
i vale 6 Soma vale 21  //6ª Iteração
i vale 7 Soma vale 28  //7ª Iteração
i vale 8 Soma vale 36  //8ª Iteração
i vale 9 Soma vale 45  //9ª Iteração
i vale 10 Soma vale 55  //10ª Iteração

When you do soma = soma + i; you increment the sum value by adding what you previously had.

For example, when i = 3 and soma = 3 , then sum will become valid 6 because soma recebe soma (que vale 3) + i (que vale 3)

I recommend that you start by taking a piece of paper and pencil and writing down the value of each iteration within the for loop. This way you will understand what happens in each iteration.

Well come on, you're looping with the variable int i which in the first iteration of for has the value of 1. You also assumed that int soma has a value of 0. Your loop will run until int i reaches the value of 10, remembering that you started it as 1 for (i = 1; your loop will run while int i is less than or equal to 10 i <= 10;

Let's now simulate your loop step by step.

First Iteration

sum = sum + i; Soma = 0 | int i = 1 | Resultado = 1

Second Iteration

sum = sum + i; Soma = 1 | int i = 2 | Resultado = 3

Third Iteration

sum = sum + i; Soma = 3 | int i = 3 | Resultado = 6

Fourth Iteration

sum = sum + i; Soma = 6 | int i = 4 | Resultado = 10

Fifth Iteration

sum = sum + i; Soma = 10 | int i = 5 | Resultado = 15

Sixth Iteration

sum = sum + i; Soma = 15 | int i = 6 | Resultado = 21

Seventh Iteration

sum = sum + i; Soma = 21 | int i = 7 | Resultado = 28

Eighth Iteration

sum = sum + i; Soma = 28 | int i = 8 | Resultado = 36

Ninth Iteration

sum = sum + i; Soma = 36 | int i = 9 | Resultado = 45

Tenth Iteration

sum = sum + i; Soma = 45 | int i = 10 | Resultado = 55

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int i, soma = 0;

    for (i = 1; i <= 10; i++) 
    {  
        soma = soma + i;
    }

    printf("Soma %d\n", soma);
    getchar();
    return 0;
}