7-1

7-1.c

#include <stdio.h>

void plus_minus (int *a, int *b) {
    int a_ori = *a,
        b_ori = *b;

    *a = a_ori + b_ori;
    *b = a_ori - b_ori;
}

int main () {
    int a = 1, b = 2;

    plus_minus(&a, &b);
    printf("%d, %d\n", a, b);

    return 0;
}