Hmmm, double check my work because I must be doing something wrong! I'm looking to evaluate -(d^2)/(dxdy) of F, where F = ax^2+bxy+cy^2 So for dF/dx, I get 2ax+by. For dF/dy, I get bx+2cy. So, multiplying both together and accounting for the negative sign: -[(2ax+by)*(bx+2cy)]= -[2abx^2+4acxy+b^2xy+2bcy^2] The answer is supposed to be -b according to the solution manual (that doesn't show the solution :p) Am I missing something here? Thanks!
Do not multiply, but apply each differential sequentially. So after finding the first differential, do the second on the resulting expression. Order is not critical.