The steady, surface temperature of a lake changes from point to point and is given to you as a function T(x, y) (assume the surface of the lake is in the (x, y) plane). If you attach a thermometer to a boat and take a path across the lake surface defined by the trajectory (x(t), y(t)) = (bi(t), b2(t)), find an expression for the rate of change of the thermometer temperature in terms of the lake temperature.