11.2 The Definition of Differentiability 1.

(a) [

COSX Df(x,y) = ~ °x 1 .

-siny

(b) Df(8,t,u,V)= [ -2t 8 °8 2vu uO].

(c) Df(t)_- [ -l/(ll/+t t)2 ] .

(d) Df(r ()) = [c~s() -rSin()].

, 8m () r cos ()

4.

(b) The function f might not be differentiable when Q = 1.