Question 478143
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How do you find the largest angle of a triangle with sides 4cm, 7cm and 9cm?
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<pre>
The largest angle is opposite to the longest side, which is 9 cm.


To find its measure  {{{alpha}}},  use the cosine law

    {{{9^2}}} = {{{4^2 + 7^2 - 2*4*7*cos(alpha)}}},

    cos(alpha) = {{{(4^2+7^2-9^2)/(2*4*7)}}} = -0.285714286.    


    {{{alpha}}} = arccos(-0.285714286) = 106.60 degrees  (approximately).    <<<---===  <U>ANSWER</U>
</pre>

Solved.