//chapter 7
printf("\n");
fcre=2.5*10^6;
fcrf=8.5*10^6;
Nmaxe=(fcre)^2/81;
Nmaxf=(fcrf)^2/81;
printf("the Nmax for e layer is %g /m^3",Nmaxe);
printf("\n the Nmax for f layer is %g /m^3",Nmaxf);