A quick way to subtract two square numbers is to use the rule  For example:  ~$a^{2}-b^{2}=\left( a+b\right)\left( a-b\right)$  $26^{2}-16^{2}=\left( 26+16\right)\left( 26-16\right)=42\times10=420$  Use this rule to work out $31^{2}-29^{2}$. (No marks will be award if you do not use the rule.) 
Overseas Competition->Knowledge Point->Calculation Modules->Operations through Formulas-> Difference of Two Squares Formula->Applying Difference of Two Squares Formula Reversely