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finding a value sin A=?   cos A = -(3/5) 90 degrees < A < 180 degrees  

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Given that cos(A) = -3/5

We need to find sin(A) suchthat A is in the second quadrant.

First let us calculate the value of sin(A).

We know that...

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giorgiana1976 | Student

The angle A is in the 2nd quadrant, so the value of the function sine is positive.

We'll apply the fundamental formula of trigonometry:

(sin A)^2 + (cos A)^2  =1

(sin A)^2 = 1  - (cos A)^2

(sin A)^2 = 1  - (-3/5)^2

(sin A)^2 = 1 - 9/25

(sin A)^2 = 16/25

sin A = 4/5

We'll keep only the positive value, since we've established that the value of the sine function is positive in the 2nd quadrant.

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