My problem solved

with special thanks to @acer :))

@Doug Meade 

thank you for your time

I use Maple17

this is my hole progrogram:

restart;

g := 5/3;

a := .1;

q := 0.1;

b:=8;

D1 := (3*g-4)/(g-1);

                               3

                               -

                               2

D2 := (9/4)*a-1/(b*(g-1));

                          0.0375000000

R := (3*D1*a+4*D2)*P(x)/((1-q*S(x))*D2);

                        16.00000000 P(x)

                        ----------------

                         1 - 0.01 S(x)  

W := sqrt(3*a*D1*(1-q*S(x))/(3*D1*a+4*D2));

                                               (1/2)

           (0.7500000000 - 0.007500000000 S(x))     

V := D2*sqrt(3*a*(1-q*S(x))/(D1*(3*D1*a+4*D2)));

                                                      (1/2)

     0.0375000000 (0.3333333333 - 0.003333333333 S(x))     

odes := D2*(diff(P(x), x))/((3*D1*a+4*D2)*P(x))-q*(diff(S(x), x))/(q*S(x)-1)-3*a*D1*cot(x)/(3*D1*a+4*D2) = 0, diff(S(x), `$`(x, 2))+(diff(S(x), x))*cot(x)+4*Pi*(3*D1*a+4*D2)*P(x)/((q*S(x)-1)*D2) = 0;

              / d      \        / d      \                         

0.06250000000 |--- P(x)|   0.01 |--- S(x)|                         

              \ dx     /        \ dx     /                         

------------------------ - --------------- - 0.7500000000 cot(x) = 

          P(x)              0.01 S(x) - 1                          

  0, 

  / d  / d      \\   / d      \          64.00000000 Pi P(x)    

  |--- |--- S(x)|| + |--- S(x)| cot(x) + ------------------- = 0

  \ dx \ dx     //   \ dx     /             0.01 S(x) - 1       

ics := P((1/2)*Pi) = 1, S((1/2)*Pi) = -1, (D(S))((1/2)*Pi) = 0;

            /1   \       /1   \           /1   \    

           P|- Pi| = 1, S|- Pi| = -1, D(S)|- Pi| = 0

            \2   /       \2   /           \2   /    

sol := dsolve({ics, odes}, numeric);

proc(x_rkf45)  ...  end;

with(plots);

odeplot(sol, [x, (3*D1*a+4*D2)*P(x)/((1-q*S(x))*D2)], .5 .. (1/2)*Pi, tickmarks = [[seq((1/30)*i*Pi = (180*i*(1/30))*`°`, i = 1 .. 20)], default]);

********************************************************

with this program i can only plot one parametr(b=8) but i need 4 plots With values (b=8,9,10,11) and I don't know how to use loop order. I will appreciate if you help me on this

restart;g := 5/3;                               5                               -                               3a := .1;restart;g := 5/3;                               5                               -                               3a := .1;

@Adri van der Meer thank you :)

@Markiyan Hirnyk  Thankyou for your time ;)

thank you, I have this item on my Mape like yours, but surprisingly it shows beta like this: b not ß

do you think what is the problem is there any way I can change its appearance? 

P.S: when I choose to convert to 2_D math mode it shows b again :"(

@Preben Alsholm yeah you right

I made a mistake because when i write pi or Pi the maple show π symbol wich i thought it understand what i mean but in calculations it only consider Pi as the real pi!

thank you so much

@Carl Love Thank you so much my problem solved :***

P.S: I work with maple17 and in this version there is no difference between pi and Pi