Question 1167410
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<pre>

Apply the cosine law

    x^2 + y^2 - 2xy*cos(a) = x^ 2 + y^2 + xy,

where "a" is the angle between the sides x and y.



It gives, after reducing

    -2xy*cos(a) = xy

     cos(a) = {{{-1/2}}}

     a = {{{2pi/3}}} = 120 degrees.    <U>ANSWER</U>
</pre>

Solved.