Calculating elasticity of substitution help

[bart11]

Calculate the elasticity of substitution between y and x for F(x,y) = 10x^2 + 15y^2

I was able to calculate the Marginal Rate of Substitution as 20x/30y but I'm not sure how to proceed past that. The answer in the book is -1. Any and all help appreciated!

 

[Dustinsfl]

Calculate the elasticity of substitution between y and x for F(x,y) = 10x^2 + 15y^2

I was able to calculate the Marginal Rate of Substitution as 20x/30y but I'm not sure how to proceed past that. The answer in the book is -1. Any and all help appreciated!

denote rho as the power.

Then elasticity of substitution is

$\dfrac{1}{1-\rho}$

If the power is different, you would have to use the formula.

 

[bart11]

denote rho as the power.

Then elasticity of substitution is

$\dfrac{1}{1-\rho}$

If the power is different, you would have to use the formula.

Sorry but I'm not sure if I follow. Rho? And I believe the book taught us using the formula so I may be a little confused. Thanks for the help!