@taro 

the f(y) is the solution of the ode as:

F_bbbb := evalf(rhs(dsolve(c1*F(y)+c2*(diff(F(y), `$`(y, 2)))+c3*(diff(F(y), `$`(y, 4))) = 2*c4*(diff(w_b1, y))*(diff(w_b2, y))+c5*(diff(w_b1, `$`(y, 2)))*w_b2+c5*(diff(w_b2, `$`(y, 2)))*w_b1, F(y))))

it is to form as below:

-0.2126639710e-354*(-29.5456666e356*exp(-4.150297840*y)*cos(3.641492391*y)+...

@Rouben Rostamian  

function f(y) is the output of an ode. the form of this function is (example) a*10^354*(b*10^-356*sin(y)), but the simple form is a*b/100 *sin(y).  The main question is : why maple dont simplify form of function f(y) ?

@Preben Alsholm  ohhh yesss. i try it in maple 16, 17 and 18:))

thank you very muchh.

@ThU thank you very muchhhh