Solve for y: Determining the Measure of Angle y

[zeion]

Homework Statement

Given cos(5pi/9) = siny
Determine the measure of angle y.

Homework Equations

The Attempt at a Solution

5pi/9 is in the second quadrant therefore the cos measure of it is negative. We want to find a sin measure that is also negative. This happens in the third and fourth quadrants.

Then y is (5pi/9) + (pi/2) or (5pi/9) + pi.

The textbook answer gives y = (5pi/9) - (pi/2) which is in the first quadrant.
The question would need to be cos(5pi/9) = -siny for that to happen.

 

[tiny-tim]

hi zeion!

(have a pi: π )

Then y is (5pi/9) + (pi/2) or (5pi/9) + pi.

The textbook answer gives y = (5pi/9) - (pi/2) which is in the first quadrant.
The question would need to be cos(5pi/9) = -siny for that to happen.

that's right

maybe by "measure" they mean "magnitude" …

Determine the measure of angle y.

what exactly was that part of the question?