<PRE><B>Question 1137907</B>
.


See the lesson

&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;- &lt;A HREF=https://www.algebra.com/algebra/homework/Trigonometry-basics/Addition-and-subtraction-formulas.lesson&gt;Addition and subtraction formulas&lt;/A&gt;

in this site and find the proof there.



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