<span class="base"><span class="strut" style="height:0.43056em;vertical-align:0em;"></span><span class="mord mathnormal">x</span></span></span></span> is a function of time <span class="katex"><span class="katex-mathml"><math><semantics><mrow><mi>x</mi><mo stretchy="false">(</mo><mi>t</mi><mo stretchy="false">)</mo><mtext>,</mtext></mrow><annotation encoding="application/x-tex">x(t)\htmlClass{math-punctuation}{\text{,}}</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span 