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*A*= [tex]\frac{-1}{\sqrt{5}}[/tex] where

*A*is more than 180 degrees and less than 270 degrees. Find the value of cos(-

*A*).

Without using Calculator,

Since cos(-

*A*) = cos

*A*, and that

*A*is in the 3rd quadrant, then after solving for the hypotenuse, adjacent and opposite, I got:

**[tex]\frac{-2}{\sqrt{5}}[/tex]**

With Calculator,

*A*= Inverse Sin([tex]\frac{-1}{\sqrt{5}}[/tex]) = -26.57 (4 T.C.)

Subst -26.57 into cos(-

*A*), I got:

**[tex]\frac{2}{\sqrt{5}}[/tex]**

One is positive, another is negative. Which is which?