The angle \theta_1θ 1
​
theta, start subscript, 1, end subscript is located in Quadrant \text{IV}IVstart text, I, V, end text, and \cos(\theta_1)=\dfrac{9}{19}cos(θ
1
​
)=
19
9
​
cosine, left parenthesis, theta, start subscript, 1, end subscript, right parenthesis, equals, start fraction, 9, divided by, 19, end fraction .
What is the value of \sin(\theta_1)sin(θ
1
​
)sine, left parenthesis, theta, start subscript, 1, end subscript, right parenthesis?
Express your answer exactly.
\sin(\theta_1)=sin(θ
1
​
)=sine, left parenthesis, theta, start subscript, 1, end subscript, right parenthesis, equals

I guess that we know that the angle θ is in the fourth quadrant, and we know that:

cos(θ) = 9/19

now we want to find the value of sin(θ).

To do it, we can remember that for a point (x, y), such that we can define an angle β between the positive x-axis and a ray that connects the origin with the point (x, y), we can write the relations:

tan(β) = x/y

sin(β) = y/√(x^2 + y^2)

cos(β) = x/√(x^2 + y^2)

Because the angle is in the fourth quadrant, we know that:

x > 0

y < 0.

And we also know that:

cos(θ) = 9/19

then we have:

x = 9

√(x^2 + y^2) = √(9^2 + y^2) = √(81 + y^2) = 19

Solving the above equation we can find the value of y, that we need to remember, is negative:

√(81 + y^2) = 19

81 + y^2 = 19^2

y^2 = 19^2 - 81 = 280

y = √280 = -16.7

Now that we know the value of y, we can replace that in the sine equation to get:

sin(θ) = -16.7/19 = -0.879

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