Answer 1

add i to x
the same as
x = x + i

Answer 2

badges are nothing man.

Answer 3

x+=i is a reduction of x = x + i
for example:
int x=0;
for(int i=0; i<3; i++)
x+=i; //x=x+i
/*
1- x=0+0=0
2- x=0+1=1
3- x=1+2=3
*/
cout<<x; //output 3