I am given that the summation from i = 1 to 3 = 1

I am to find x sub 1, xsub2, xsub3 so that the summation from i = 1 to 3 of x sub i ^2 is at a minimum. Then I am suppose to come up for a general formula based on my findings. If anyone is willing to give me a push in the right direction it would be much appreciated! Frostking