Let n∈N, let U⊆Rn be open, let f:U→R be a C1 map, let i∈{1,…,n}, and let x=(x1,…,xn)∈U. Let a,b∈R with a<b, and assume that
{(x1,…,xi−1,t,xi+1,…,xn):t∈[a,b]}⊆U.
Define g:[a,b]→R by
g(t)=f(x1,…,xi−1,t,xi+1,…,xn).
Then g is continuous on [a,b], differentiable at every t∈(a,b), and
g′(t)=∂xi∂f(x1,…,xi−1,t,xi+1,…,xn)
for every t∈(a,b).