
Using the two-variable Taylor series [say ( 2.7)] prove the following "second derivative tests" for maximum or minimum points of functions of two variables. If fx=fy=0 at (a,b), then (a,b) is a minimum point if at (a,b),fxx>0,fyy>0, and fxxfyy>f2xy;
(a,b) is a maximum point if at (a,b),fxx<0,fyy<0, and fxxfyy>f2xy; (a,b) is neither a maximum nor a minimum point if fxxfyy<f2xy. (Note that this includes fxxfyy<0, that is, fxx and fyy of opposite sign.)
Hint: Let fxx=A,fxy=B,fyy=C; then the second derivative terms in the Taylor series are Ah2+2Bhk+Ck2; this can be written A(h+Bk/A)2+(C−B2/A)k2 Find out when this expression is positive for all small h,k [that is, all (x,y) near (a,b)]; also find out when it is negative for all small h, k, and when it has both positive and negative values for small h, k .

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