Added 1.2.20-1.2.21; 1.3.1-1.3.2

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+ <h2>Solutions of Savchenko Problems in Physics</h2>
+ <p class="author">
+ Aliaksandr Melnichenka <br/>
+ October 2023
+ </p>
+ </header>
+
+ <h3 id="back-link"><a href="../#1.2">$\leftarrow$Back</a></h3>
+
+ <h3> Statement </h3>
+ <p>
+ $1.2.20.$ The body starts moving from point $A$ and moves first equidistant for time $t_0$, then with the same modulo acceleration — equidistant. After what time from the beginning of the movement, the body will return to point $A$?
+ </p>
+
+ <h3>Solution</h3>
+ <p>
+ During the time $t_0$ the velocity of the body became equal to $v_0 = at_0$, and the coordinate
+$$x_0 = \frac{at_0^2}{2}$$
+It takes time $t_1$ for the body to travel the distance $x_1$:
+$$-x_1 = v_0 t - \frac{at^2}{2}$$
+Solving a quadratic equation:
+$$t_1^2 - 2t_0 t_1-t_0^2=0$$
+We choose a positive root:
+$$t_1 = (1+\sqrt{2})t_0$$
+Round trip time:
+$$T = t_0+t_1=(2+\sqrt{2})t_0$$
+
+ </p>
+
+ <h4>Answer</h4>
+ <p>
+ $$t =(2+\sqrt{2})t_0$$
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