# 什么是相关变化率？怎么求？

$2x \frac{dx}{dt}+2y\frac{dy}{dt}=0$

$8\cdot 5\pm6 \frac{dy}{dt}=0$

$\frac{dy}{dt}=\mp \frac{20}{3} .$

\begin{tikzpicture}

\draw [->] (4,0)–(2,0);

\draw [->] (4,0)–(4,-4);

\draw (2,0)–(4,-4);

\node [right] at (4,0) {$A$};

\node [below] at (4,-4) {$B$};

\node [left] at (2,0) {$C$};

\node [right] at (4,-2) {$x$};

\node [above] at (3,0) {$y$};

\node [left ] at (2.8,-2) {$z$};

\end{tikzpicture}

$2z\frac{dz}{dt}=2x\frac{dx}{dt}+2y\frac{dy}{dt}$

$2\cdot 130 \frac{dz}{dt}=2\cdot 120\cdot 60+2\cdot 50\cdot 25$