Suppose a quasi-linear preference is represented by a utility function of the form u(x)=x1+(x2,...,xn) . Show that the Hicksian demand functions for goods 2,...,n are

Suppose a quasi-linear preference is represented by a utility function of the form u(x)=x1+ϕ(x2,...,xn). Show that the Hicksian demand functions for goods 2,...,n are independent of utility *WITHOUT* assuming this function is differentiable, i.e. use the definition of quasilinear preference to prove.