7.1.c

#include <stdio.h>

void plus_minus(int *a, int *b) 
{
    int t1 = *a,
	    t2 = *b;

	*a = t1 + t2;
	*b = t1 - t2;
}

int main()
{
	int a = 1, b = 2;
	plus_minus(&a, &b);
	printf("%d %d\n",a,b);
	return 0;
}