Use the chain rule to calculate df(x,y) / dt for f(x,y) = sqrt (1 + x^2 + y^2) when : x = ln t and y = cos t.
I know that the chain rule is :
dz/dt = df/dx * dx/dt + df/dy * dy/dt
df/dx = x/sqrt(1 + x^2 + y^2)
just not sure where else to go from there...