JSFiddle - React, Tailwind, and code Playground

by Qwerty_Wasd

HTML

<form name="cal">
	<table>
		<tr>
			<td>a:</td>
			<td><input class="value-diff" type="text" size=30 name="a"></td>
		</tr>
		<tr>
			<td>b:</td>
			<td><input class="value-diff" type="text" size=30 name="b"></td>
		</tr>
		<tr>
			<td>U:</td>
			<td><input class="value-diff" type="text" size=30 name="U"></td>
		</tr>
	</table>
	<input type="button" value="Посчитать">
</form>

JavaScript

let opMax = {
	form: document.querySelector(`[name="cal"]`),
	input: document.getElementsByClassName(`value-diff`),
	events: {
		click: e => {
			opMax.input[2].value = Math.max(Math.max((+opMax.input[0].value|0),(+opMax.input[1].value|0)), (+opMax.input[1].value|0)**2,(+opMax.input[0].value|0)**2);
		}
	},
};
opMax.form.lastElementChild.addEventListener(`click`, opMax.events.click);