Derive the Euler-Lagrange Differential Equation (rigorously). Explain any theorems or results that you use in the derivation (e.g. don't just say "by the fundamental theorem of the calculus of variations" without telling what the fundamental theorem of the calculus of variations is and what it means). Also explain in words and/or pictures why the Euler-Lagrange Differential Equation is believable (intuitively).