4. Derive the following relationship between the Lerner index for a monopolist and the price elasticity of demand: You will need to use the formula for elasticity ($9,, = %%), and it will help to recall that, for a monopolist, MR = c at the prot-maximizing level of output. EXTENDED GUIDANCE: Begin by deriving marginal revenue for the monopolist, MR = Mfg. Then, by rearranging terms and substituting in the formula above for elasticity, rewrite that expression for MR as a function of p and 59p. (HINT: You will need to multiply and divide by 13) After that, use the fact that MR = c in equilibrium for a monopolist to set your expression equal to marginal cost. From there, you should be able to simply rearrange terms to get to the nal result