Step by step
The question
Question: find sin 2θ when cos θ = 4/5, θ in quadrant I.
We know cos θ, and the quadrant.
1 Draw the triangle
Take a point (x, y) at distance r from O.
cos θ is x over r.
So x = 4 and r = 5.
We draw it in quadrant I: a right triangle from O to the point.
2 Find the third side
Pythagoras: x² + y² = r².
4 squared is 16.
5 squared is 25.
Take away 16 on both sides: y² = 9.
The square root of 9 is 3.
Plus or minus.
In quadrant I, y is positive: above the x-axis.
So y = 3.
3 Find sin θ
Sin θ is y/r: 3/5.
So sin θ = 3/5.
4 Write the rule
The double-angle rule: sin 2θ = 2 sin θ cos θ.
Careful!
sin 2θ is not 2 times sin θ.
The 2 is inside: it doubles the angle, not the answer.
6/5 is not between −1 and 1, and a sine always is.
5 Work it out
Put in sin θ = 3/5 and cos θ = 4/5.
Tops: 2 times 3 times 4 is 24.
Bottoms: 5 times 5 is 25.
So sin 2θ = 24/25.
6 Check
Let's check with the calculator.
The reference angle is tan⁻¹ of 3/4: about 36.87°.
In quadrant I, θ is about 36.87°.
Twice that is 73.74°.
sin of it is about 0.960.
And 24/25 is 0.960.
The same!
So it is right.
Let's look back.
First: draw the triangle.
Then: find the third side.
Then: find sin θ.
Then: write the rule.
Then: work it out.
Last: check.