On Tuesday, April 23, 2013 3:10:43 PM UTC+1, oercim wrote: > Hello. I need help. Let x1,x2,...,xn be n variables and let ave(x) be aritmetic avarage of these n variables. Then, is below inequality true? How can I show if it is true? > > > > x1^2+x2^2+....+xn^2>ave(x)^2 > > > > Thanks a lot. For all n >=1 , you get a counterexample when x1 = x2 ... = xn = 0. This is definitely not the answer the lecturer or textbook wanted, but it shows that more care is needed in the question.