# Replace a variable with a function

Hello i hope anyone can help me with the following problem.

i have the following code

```
T_w,y,T_m,k_f,rho_f,U_0,h_mstar,R, alpha, delta, Phi = var('T_w y T_m k_f rho_f U_0 h_mstar R alpha delta Phi')
T=T_w+y*(-2*(T_w-T_m)/delta+rho_f*U_0*h_mstar*cos(Phi)/k_f)+y^2*((T_w-T_m)/delta^2-rho_f*U_0*h_mstar*cos(Phi)/(k_f*delta))
u=-6*U_0*R*y*(y-delta)*sin(Phi)/delta^3
integralTu=integrate(T*u,y,0,delta)
```

now i would like to differentiate integralTu with respect to x (diff(integralTu,x)), but delta must be a function of x (i first defined it as a variable because of the integration).

@tetraeder, @kcrisman: add tags?