143. Find $\lim _{x \rightarrow 0} \frac{\sin k x}{x}(k-$ a constant).
Solution. Let's make the substitution $k x=y$. From this, it follows that $y \rightarrow 0$ as $x \rightarrow 0$, and $x=y / k$. Then we get

$$
\lim _{x \rightarrow 0} \frac{\sin k x}{x}=\lim _{y \rightarrow 0} \frac{\sin y}{y / k}=\lim _{y \rightarrow 0} \frac{k \sin y}{y}=k \lim _{y \rightarrow 0} \frac{\sin y}{y}=k
$$

since $\lim _{y \rightarrow 0} \frac{\sin y}{y}=1$.