Question 1158448
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The point is in QIII, where cosine is NEGATIVE.



cos(a) is the ratio of the adjacent leg (which is -7 in this case) to the hypotenuse length;

    the sign of the ratio should be selected as a " - " (minus).



The hypotenuse length is  {{{sqrt((-7)^2 + (-3)^2)}}} = {{{sqrt(49+9)}}} = {{{sqrt(58)}}}.



Hence, the answer is  cos(a) = - {{{7/sqrt(58)}}}.
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Solved and explained.