First-order condition constrained extremum problem

Introduction: A constrained extremum problem is given by
  • Optimize z(x,y)
  • Subject to g(x,y)=k
  • Where xD1, yD2
Theorem: An extremum location (c,d), where cD1 and dD2 that is not a boundary point, satisfies the following system of equations:
  • zx(x,y)zy(x,y)=gx(x,y)gy(x,y)
  • g(x,y)=k

Remark: Do not forget to check whether the extremum is a minimum or a maximum.