u dont sub t=1
let Φ = f (tx,ty) = t^n f(x,y)
dΦ/dt = (∂f/∂tx)*(dtx/dt) + (∂f/∂ty)*(dty/dt) = n*t^(n-1) f(x,y)
(∂f/∂tx)*x + (∂f/∂ty)*y = n*t^(n-1) f(x,y)
(∂f/∂tx)*tx + (∂f/∂ty)*ty = n*t^n f(x,y)
(∂f/∂tx)*tx + (∂f/∂ty)*ty = n*f(tx,ty)
(∂f/∂u)*u + (∂f/∂v)*v = n*f(u,v)
(∂f/∂x)*x +...