I want to show that $f(x,y)=|xy|^{1/2}$ is differentiable at $(1,1)$.
I am trying to use that formula $\frac{f(h,k)-f(a,b)-Df(a,b)(h,k)}{ \|(h,k)\|}$ to get a result. but so far I haven't figured out where I am making mistake.
in this situation $(a,b)$ is $(1,1)$ $Df$ is a total derivative.
Thank you for everyone sharing their opinion.