@Carl Love 

Yes am sure its from  n..(n+1)*h

I have Tried this :

Y := (eval(F, t = (n+1)*h)-(eval(F, t = n)))/(eval(L, t = (n+1)*h)-(eval(L, t = n)));

Interactive(Y);

where,

F := int(B*eta^(-B)*t^(B-1)*exp(-(t/eta)^B)*(t-n*h), t)

and

L := int(B*eta^(-B)*t^(B-1)*exp(-(t/eta)^B), t)

But got this Error:

Complex value encountered

What do you think

Regards, 

@Carl Love 

I have tried it again as you explained and it does work .. Thank you

YES .. you can assume that n is positive integer and h is positive ..

This is what am trying to optimize (minimize) with respect to n and h:

(int(B*eta^(-B)*t^(B-1)*exp(-(t/eta)^B)*(t-n*h), t = n .. (n+1)*h))/(int(B*eta^(-B)*t^(B-1)*exp(-(t/eta)^B), t = n .. (n+1)*h))

Thanks

@Carl Love 

i Have tried your way even with n*h rather than n

but this error came:

Error, invalid input: eval received n*h, which is not valid for its 2nd argument, eqns

My actual objective is to fined the optimal values of n and h that gives 

the optimal value of this equation:

(int(B*eta^(-B)*t^(B-1)*exp(-(t/eta)^B)*(t-n*h), t = n .. (n+1)*h))/(int(B*eta^(-B)*t^(B-1)*exp(-(t/eta)^B), t = n .. (n+1)*h))

And since only the Numerator didn't integrate i thought the error is her

what do you think

Thanks 

@Kitonum 

Am trying to find the optimal values tha minimize this :

(int(B*eta^(-B)*t^(B-1)*exp(-(t/eta)^B)*(t-n*h), t = n .. (n+1)*h))/(int(B*eta^(-B)*t^(B-1)*exp(-(t/eta)^B), t = n .. (n+1)*h))

Thats way i need to have n and h as varibles to get there optimal values ..

But i do get that Erro .. and i thought the problem is in the numerator 

becuse it does not integrate .. 

So what do you think

Thank you