[proofplan] The proof computes the first variation for one admissible endpoint-moving family and then rewrites the boundary contribution in terms of the actual endpoint velocity in $\mathbb{R}^2$. The moving upper limit contributes the term $L(b,y(b),y'(b))\delta b$, while [integration by parts](/theorems/210) contributes $\partial_{y'}L(b,y(b),y'(b))h(b)$ from the vertical graph variation. The endpoint velocity is not $(\delta b,h(b))$, but rather $(\delta b,\delta y)$ with $\delta y=h(b)+y'(b)\delta b$, and this substitution produces the transversality vector. Since every tangent direction to $\Gamma$ is realised by an admissible variation, stationarity forces that vector to be orthogonal to the full tangent line of $\Gamma$. [/proofplan] [step:Introduce the endpoint and graph variation velocities] Fix an admissible variation $\varepsilon \mapsto (b_\varepsilon,Y_\varepsilon)$. Define the endpoint abscissa velocity $\delta b \in \mathbb{R}$ by \begin{align*} \delta b=\frac{d}{d\varepsilon}\Big|_{\varepsilon=0}b_\varepsilon. \end{align*} Define the graph variation field $h:[a,b]\to\mathbb{R}$ by \begin{align*} h(x)=\frac{\partial Y_\varepsilon}{\partial\varepsilon}(0,x). \end{align*} Because $(\varepsilon,x)\mapsto Y_\varepsilon(x)$ is $C^2$ on $(-\varepsilon_0,\varepsilon_0)\times U$, the function $h$ is $C^1$ on $[a,b]$ and \begin{align*} h'(x)=\frac{\partial}{\partial\varepsilon}\Big|_{\varepsilon=0}\partial_xY_\varepsilon(x) \end{align*} for every $x\in[a,b]$. Because $Y_\varepsilon(a)=y(a)$ for all $\varepsilon$, differentiating at $\varepsilon=0$ gives \begin{align*} h(a)=0. \end{align*} Define the endpoint ordinate velocity $\delta y \in \mathbb{R}$ by \begin{align*} \delta y=\frac{d}{d\varepsilon}\Big|_{\varepsilon=0}Y_\varepsilon(b_\varepsilon). \end{align*} The chain rule applied to the $C^1$ map $(\varepsilon,x)\mapsto Y_\varepsilon(x)$ at $(0,b)$ gives \begin{align*} \delta y=h(b)+y'(b)\delta b. \end{align*} Thus the endpoint velocity is \begin{align*} \tau=(\delta b,\delta y)=(\delta b,h(b)+y'(b)\delta b). \end{align*} [guided] The variation has two different first-order quantities at the right endpoint, and it is important not to confuse them. First, the endpoint moves horizontally with velocity \begin{align*} \delta b=\frac{d}{d\varepsilon}\Big|_{\varepsilon=0}b_\varepsilon. \end{align*} Second, the graph itself changes vertically at a fixed abscissa. We encode that fixed-abscissa vertical variation by the map $h:[a,b]\to\mathbb{R}$ defined by \begin{align*} h(x)=\frac{\partial Y_\varepsilon}{\partial\varepsilon}(0,x). \end{align*} The left endpoint is fixed, so $Y_\varepsilon(a)=y(a)$ for every $\varepsilon$. Differentiating this identity with respect to $\varepsilon$ gives \begin{align*} h(a)=0. \end{align*} The ordinate of the moving endpoint is not simply $Y_\varepsilon(b)$. It is $Y_\varepsilon(b_\varepsilon)$, so its derivative has two contributions: the graph changes with $\varepsilon$, and the evaluation point $b_\varepsilon$ moves along the original graph. Define \begin{align*} \delta y=\frac{d}{d\varepsilon}\Big|_{\varepsilon=0}Y_\varepsilon(b_\varepsilon). \end{align*} By the chain rule, \begin{align*} \delta y=\frac{\partial Y_\varepsilon}{\partial\varepsilon}(0,b)+\partial_xY_0(b)\delta b. \end{align*} Since $Y_0|_{[a,b]}=y$, this becomes \begin{align*} \delta y=h(b)+y'(b)\delta b. \end{align*} Therefore the velocity of the endpoint as a point of the plane is \begin{align*} \tau=(\delta b,\delta y)=(\delta b,h(b)+y'(b)\delta b). \end{align*} This identity is the bridge between the variational boundary term, which naturally contains $h(b)$, and the geometric constraint, which is stated in terms of tangent vectors to $\Gamma$ in $\mathbb{R}^2$. [/guided] [/step] [step:Differentiate the functional and isolate the boundary terms] Define the auxiliary functions \begin{align*} L_y:U\times\mathbb{R}\times\mathbb{R}\to\mathbb{R} \end{align*} and \begin{align*} L_{y'}:U\times\mathbb{R}\times\mathbb{R}\to\mathbb{R} \end{align*} by \begin{align*} L_y(x,z,p)=\partial_yL(x,z,p) \end{align*} and \begin{align*} L_{y'}(x,z,p)=\partial_{y'}L(x,z,p). \end{align*} After shrinking $\varepsilon_0>0$ if necessary, the continuity of $\varepsilon\mapsto b_\varepsilon$ and the inequality $a<b_0=b$ ensure that $a<b_\varepsilon$ for all $|\varepsilon|<\varepsilon_0$, so each interval $[a,b_\varepsilon]$ has the intended orientation. The map $(\varepsilon,x)\mapsto L(x,Y_\varepsilon(x),\partial_xY_\varepsilon(x))$ is $C^1$ on a neighbourhood of $\{0\}\times[a,b]$, because $L\in C^2$ and $(\varepsilon,x)\mapsto Y_\varepsilon(x)$ is $C^2$. The one-dimensional Leibniz rule for a $C^1$ integrand with $C^1$ moving upper limit therefore gives \begin{align*} I'(0)=\int_{[a,b]}\left(L_y(x,y(x),y'(x))h(x)+L_{y'}(x,y(x),y'(x))h'(x)\right)\,d\mathcal{L}^1(x)+L(b,y(b),y'(b))\delta b. \end{align*} Since $L\in C^2$ and $y\in C^2([a,b])$, the function \begin{align*} x\mapsto L_{y'}(x,y(x),y'(x)) \end{align*} is $C^1$ on $[a,b]$. [Integration by parts](/theorems/2098) on $[a,b]$ gives \begin{align*} \int_{[a,b]}L_{y'}(x,y(x),y'(x))h'(x)\,d\mathcal{L}^1(x)=L_{y'}(b,y(b),y'(b))h(b)-L_{y'}(a,y(a),y'(a))h(a)-\int_{[a,b]}\frac{d}{dx}L_{y'}(x,y(x),y'(x))h(x)\,d\mathcal{L}^1(x). \end{align*} Using $h(a)=0$, this becomes \begin{align*} I'(0)=\int_{[a,b]}\left(L_y(x,y(x),y'(x))-\frac{d}{dx}L_{y'}(x,y(x),y'(x))\right)h(x)\,d\mathcal{L}^1(x)+L_{y'}(b,y(b),y'(b))h(b)+L(b,y(b),y'(b))\delta b. \end{align*} [/step] [step:Use stationarity to cancel the interior Euler-Lagrange term] By the fixed-endpoint interior variation richness assumed in the statement, for every $\phi\in C_c^1((a,b))$ the family $b_\varepsilon=b$ and $Y_\varepsilon=y+\varepsilon\phi$ is admissible. Applying stationarity to these variations gives the weak Euler-Lagrange identity \begin{align*} \int_{[a,b]}\left(L_y(x,y(x),y'(x))-\frac{d}{dx}L_{y'}(x,y(x),y'(x))\right)\phi(x)\,d\mathcal{L}^1(x)=0 \end{align*} for every $\phi\in C_c^1((a,b))$. Since the integrand is continuous on $(a,b)$, the fundamental lemma of the [calculus of variations](/page/Calculus%20of%20Variations) gives the Euler-Lagrange equation along $y$: \begin{align*} L_y(x,y(x),y'(x))-\frac{d}{dx}L_{y'}(x,y(x),y'(x))=0 \end{align*} for every $x\in(a,b)$. Substituting this identity into the [first variation formula](/theorems/2728) gives, for every admissible endpoint-moving variation, \begin{align*} 0=I'(0)=L_{y'}(b,y(b),y'(b))h(b)+L(b,y(b),y'(b))\delta b. \end{align*} [/step] [step:Rewrite the boundary expression using the actual endpoint velocity] From the endpoint velocity relation, \begin{align*} h(b)=\delta y-y'(b)\delta b. \end{align*} Substituting this into the boundary identity yields \begin{align*} 0=L_{y'}(b,y(b),y'(b))(\delta y-y'(b)\delta b)+L(b,y(b),y'(b))\delta b. \end{align*} Collecting the coefficients of $\delta b$ and $\delta y$ gives \begin{align*} 0=\left(L(b,y(b),y'(b))-y'(b)L_{y'}(b,y(b),y'(b))\right)\delta b+L_{y'}(b,y(b),y'(b))\delta y. \end{align*} Equivalently, \begin{align*} \left(L(b,y(b),y'(b))-y'(b)\partial_{y'}L(b,y(b),y'(b)),\partial_{y'}L(b,y(b),y'(b))\right)\cdot(\delta b,\delta y)=0. \end{align*} Since $(\delta b,\delta y)=\tau$, the displayed orthogonality holds for the endpoint velocity of the chosen admissible variation. [guided] For the chosen admissible variation, stationarity gives $I'(0)=0$. The differentiated functional and the integration by parts computation give \begin{align*} I'(0)=\int_{[a,b]}\left(L_y(x,y(x),y'(x))-\frac{d}{dx}L_{y'}(x,y(x),y'(x))\right)h(x)\,d\mathcal{L}^1(x)+L_{y'}(b,y(b),y'(b))h(b)+L(b,y(b),y'(b))\delta b. \end{align*} The fixed-endpoint interior variations imply the Euler-Lagrange equation \begin{align*} L_y(x,y(x),y'(x))-\frac{d}{dx}L_{y'}(x,y(x),y'(x))=0 \end{align*} for every $x\in(a,b)$, and the integrand is continuous on $[a,b]$. Hence the interior integral vanishes, and stationarity reduces the first variation to the boundary identity \begin{align*} 0=L_{y'}(b,y(b),y'(b))h(b)+L(b,y(b),y'(b))\delta b. \end{align*} This expression is not yet the desired transversality condition because the geometric constraint is imposed on the moving endpoint as a point of $\mathbb{R}^2$. The relevant endpoint velocity is \begin{align*} \tau=(\delta b,\delta y), \end{align*} not $(\delta b,h(b))$. The chain rule for the moving endpoint gives \begin{align*} \delta y=h(b)+y'(b)\delta b. \end{align*} Solving for $h(b)$ gives \begin{align*} h(b)=\delta y-y'(b)\delta b. \end{align*} Substituting this into the boundary identity gives \begin{align*} 0=L_{y'}(b,y(b),y'(b))(\delta y-y'(b)\delta b)+L(b,y(b),y'(b))\delta b. \end{align*} Now group the coefficient of the horizontal endpoint velocity $\delta b$ and the coefficient of the vertical endpoint velocity $\delta y$: \begin{align*} 0=\left(L(b,y(b),y'(b))-y'(b)L_{y'}(b,y(b),y'(b))\right)\delta b+L_{y'}(b,y(b),y'(b))\delta y. \end{align*} Since $L_{y'}=\partial_{y'}L$ by definition, this is exactly the dot product identity \begin{align*} \left(L(b,y(b),y'(b))-y'(b)\partial_{y'}L(b,y(b),y'(b)),\partial_{y'}L(b,y(b),y'(b))\right)\cdot(\delta b,\delta y)=0. \end{align*} The key point is that the term $-y'(b)\partial_{y'}L$ appears because a horizontal displacement of the endpoint also changes the endpoint ordinate along the original curve by $y'(b)\delta b$. [/guided] [/step] [step:Apply the realisation hypothesis for tangent directions] For every admissible variation, the endpoint curve condition \begin{align*} (b_\varepsilon,Y_\varepsilon(b_\varepsilon))\in\Gamma \end{align*} implies that its endpoint velocity $\tau$ lies in $T_{(b,y(b))}\Gamma$. The preceding step proves the required dot product identity for every tangent vector that is realised as such an endpoint velocity. By hypothesis, every vector in $T_{(b,y(b))}\Gamma$ is realised by some admissible endpoint-moving variation. Therefore, for every $\tau\in T_{(b,y(b))}\Gamma$, \begin{align*} \left(L(b,y(b),y'(b))-y'(b)\partial_{y'}L(b,y(b),y'(b)),\partial_{y'}L(b,y(b),y'(b))\right)\cdot\tau=0. \end{align*} This is the asserted transversality condition. [/step]