Let f : be given by f ( ) = ( ). Show that f is differentiable at = (0,0,0) and find df( ).

I figured to use the definition so:

f(X) = f( ) + df( )(X- ) + o(|x- |) as X

f( ) goes to 0 and X- is just X

So I'm left with:

f(X) = df( )(X) + o(|X|) as X

But I'm not sure how to find df( )

Doing so would that f is differentiable at and find it at the same time, yes?