Added translation using Weblate (Russian)

help/ru/variables.page

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+<page xmlns="http://projectmallard.org/1.0/"
+      xmlns:its="http://www.w3.org/2005/11/its"
+      type="guide" style="2column task"
+      id="variables">
+
+  <info>
+    <link type="guide" xref="index#formulae"/>
+
+    <desc>Easily adjustable parameters</desc>
+  </info>
+  <title>Variables and sliders</title>
+
+  <p>
+    To declare a variable, type an equation of the form
+    <code><em>variable</em> = <em>value</em></code> into an empty formula box, where
+    <code><em>variable</em></code> is a single Latin or Greek letter, and
+    <code><em>value</em></code> is an expression which must not depend on position, but
+    may depend on preceding variables.
+  </p>
+
+  <p>
+    If <code><em>value</em></code> is a literal integer or decimal number, Plots will
+    display an adjustable slider control, allowing the variable to be adjusted
+    interactively. The upper and lower limits of the slider can be edited using the
+    adjacent fields.
+  </p>
+</page>