Question

Need help finding error using price-demand equation to find E(x) at x=400.
E(x)= g(x)/xg'(x)
p=g(x)=50-2x^1/2

g'(x)=-x^-1/2

E(400)=50-2^1/2 / x(-x^-1/2)
=50-2x^1/2 / -x^1/2
=50-2x^1/2 / -x^1/2
=50-[2(400^1/2)] / -400^1/2
=10/-20
=-1/2

Actual answer is 1/2. Not sure where I missed the change of sign.