... For instance, one can write . ... This is not an equation but an identity , an expression true for any three numbers (a,b,c). ... Identities do not add any information about the quantities which they contain, because they are true whatevers their values may be. They are however useful in reshuffling equations to new, cleaner forms. The identity written on top is actually one of the basic properties of numbers ("the distributive law"). ... a 2 + 2ab + b 2 . ... Again, the two last identities . ...