Function to find the length of the direct common tangent
static void lengtang ( double r1 , double r2 ) { Console . WriteLine ( " The ▁ length ▁ of ▁ the ▁ " + " direct ▁ common ▁ tangent ▁ is ▁ " + ( 2 * Math . Sqrt ( r1 * r2 ) ) ) ; }