desigualtats

( x+(-y) )^{2}+( y+(-z) )^{2}+( z+(-x) )^{2} >] 0 <==> x^{2}+y^{2}+z^{2} >] xy+yz+zx




Si a_{1}·...·a_{n}=1  ==> (1+a_{1})·...·(1+a_{n}) >] 2^{n}
( 1+(-1)(a_{k})^{(1/2)} )^{2} >] 0