Function to find the length of the direct common tangent
< ? php function lengtang ( $ r1 , $ r2 , $ d ) { echo " The ▁ length ▁ of ▁ the ▁ direct ▁ common ▁ tangent ▁ is ▁ " , sqrt ( pow ( $ d , 2 ) - pow ( ( $ r1 - $ r2 ) , 2 ) ) ; }