if cos a = -5/6 , what is tan a?

There are two possibilities:

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Thanks Alan, it was pi/2 < a < pi so -sqrt(11)/5 should be correct.


So the process is, calculate the third unknown side using pythagoras and then tan a = opposite side / closest side. Correct me if I'm wrong.

English is not my native language, dont know the correct term for the sides in a triangle.

"So the process is, calculate the third unknown side using pythagoras and then tan a = opposite side / closest side. "

This is correct.  

The "closest" side is called the "adjacent" side.

The "opposite" side is just that.

The longest side is called the "hypotenuse".

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