printf("\t example 7.1 \n");
PT=1; // square pitch,in
do=0.75; // outer diameter,in
de=((4*(PT^2-(3.14*do^2/4)))/(3.14*do));
printf("\t equivalent diameter is : %.2f in \n",de);
De=(de/12); // ft
printf("\t De is : %.3f in \n",De);
//end