Answer
The power lost in the lines as heat is:
$$P_L=\frac{P_T^2R_L}{V^2}$$
Work Step by Step
Find the current in the transmission lines from P=IV, where P is the power transmitted to the user and V is the delivered voltage.
$$P_T=IV$$
$$I=\frac{P_T}{V}$$
The power lost in the lines as heat is given by $I^2R$, where R is the resistance of the power line.
$$P_L=I^2R_L=(\frac{P_T}{V})^2R_L=\frac{P_T^2R_L}{V^2}$$
