# 7.3 Double-angle, half-angle, and reduction formulas  (Page 4/8)

 Page 4 / 8

Given that $\text{\hspace{0.17em}}\mathrm{sin}\text{\hspace{0.17em}}\alpha =-\frac{4}{5}\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}\alpha \text{\hspace{0.17em}}$ lies in quadrant IV, find the exact value of $\text{\hspace{0.17em}}\mathrm{cos}\text{\hspace{0.17em}}\left(\frac{\alpha }{2}\right).$

$-\frac{2}{\sqrt{5}}$

## Finding the measurement of a half angle

Now, we will return to the problem posed at the beginning of the section. A bicycle ramp is constructed for high-level competition with an angle of $\text{\hspace{0.17em}}\theta \text{\hspace{0.17em}}$ formed by the ramp and the ground. Another ramp is to be constructed half as steep for novice competition. If $\text{\hspace{0.17em}}\mathrm{tan}\text{\hspace{0.17em}}\theta =\frac{5}{3}\text{\hspace{0.17em}}$ for higher-level competition, what is the measurement of the angle for novice competition?

Since the angle for novice competition measures half the steepness of the angle for the high level competition, and $\text{\hspace{0.17em}}\mathrm{tan}\text{\hspace{0.17em}}\theta =\frac{5}{3}\text{\hspace{0.17em}}$ for high competition, we can find $\text{\hspace{0.17em}}\mathrm{cos}\text{\hspace{0.17em}}\theta \text{\hspace{0.17em}}$ from the right triangle and the Pythagorean theorem so that we can use the half-angle identities. See [link] .

We see that $\text{\hspace{0.17em}}\mathrm{cos}\text{\hspace{0.17em}}\theta =\frac{3}{\sqrt{34}}=\frac{3\sqrt{34}}{34}.\text{\hspace{0.17em}}$ We can use the half-angle formula for tangent: $\text{\hspace{0.17em}}\mathrm{tan}\text{\hspace{0.17em}}\frac{\theta }{2}=\sqrt{\frac{1-\mathrm{cos}\text{\hspace{0.17em}}\theta }{1+\mathrm{cos}\text{\hspace{0.17em}}\theta }}.\text{\hspace{0.17em}}$ Since $\text{\hspace{0.17em}}\mathrm{tan}\text{\hspace{0.17em}}\theta \text{\hspace{0.17em}}$ is in the first quadrant, so is $\text{\hspace{0.17em}}\mathrm{tan}\text{\hspace{0.17em}}\frac{\theta }{2}.\text{\hspace{0.17em}}$ Thus,

We can take the inverse tangent to find the angle: $\text{\hspace{0.17em}}{\mathrm{tan}}^{-1}\left(0.57\right)\approx {29.7}^{\circ }.\text{\hspace{0.17em}}$ So the angle of the ramp for novice competition is $\text{\hspace{0.17em}}\approx {29.7}^{\circ }.$

Access these online resources for additional instruction and practice with double-angle, half-angle, and reduction formulas.

## Key equations

 Double-angle formulas Reduction formulas $\begin{array}{l}{\mathrm{sin}}^{2}\theta =\frac{1-\mathrm{cos}\left(2\theta \right)}{2}\\ {\mathrm{cos}}^{2}\theta =\frac{1+\mathrm{cos}\left(2\theta \right)}{2}\\ {\mathrm{tan}}^{2}\theta =\frac{1-\mathrm{cos}\left(2\theta \right)}{1+\mathrm{cos}\left(2\theta \right)}\end{array}$ Half-angle formulas

## Key concepts

• Double-angle identities are derived from the sum formulas of the fundamental trigonometric functions: sine, cosine, and tangent. See [link] , [link] , [link] , and [link] .
• Reduction formulas are especially useful in calculus, as they allow us to reduce the power of the trigonometric term. See [link] and [link] .
• Half-angle formulas allow us to find the value of trigonometric functions involving half-angles, whether the original angle is known or not. See [link] , [link] , and [link] .

## Verbal

Explain how to determine the reduction identities from the double-angle identity $\text{\hspace{0.17em}}\mathrm{cos}\left(2x\right)={\mathrm{cos}}^{2}x-{\mathrm{sin}}^{2}x.$

Use the Pythagorean identities and isolate the squared term.

Explain how to determine the double-angle formula for $\text{\hspace{0.17em}}\mathrm{tan}\left(2x\right)\text{\hspace{0.17em}}$ using the double-angle formulas for $\text{\hspace{0.17em}}\mathrm{cos}\left(2x\right)\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}\mathrm{sin}\left(2x\right).$

We can determine the half-angle formula for $\text{\hspace{0.17em}}\mathrm{tan}\left(\frac{x}{2}\right)=\frac{\sqrt{1-\mathrm{cos}\text{\hspace{0.17em}}x}}{\sqrt{1+\mathrm{cos}\text{\hspace{0.17em}}x}}\text{\hspace{0.17em}}$ by dividing the formula for $\text{\hspace{0.17em}}\mathrm{sin}\left(\frac{x}{2}\right)\text{\hspace{0.17em}}$ by $\text{\hspace{0.17em}}\mathrm{cos}\left(\frac{x}{2}\right).\text{\hspace{0.17em}}$ Explain how to determine two formulas for $\text{\hspace{0.17em}}\mathrm{tan}\left(\frac{x}{2}\right)\text{\hspace{0.17em}}$ that do not involve any square roots.

$\text{\hspace{0.17em}}\frac{1-\mathrm{cos}\text{\hspace{0.17em}}x}{\mathrm{sin}\text{\hspace{0.17em}}x},\frac{\mathrm{sin}\text{\hspace{0.17em}}x}{1+\mathrm{cos}\text{\hspace{0.17em}}x},$ multiplying the top and bottom by $\text{\hspace{0.17em}}\sqrt{1-\mathrm{cos}\text{\hspace{0.17em}}x}\text{\hspace{0.17em}}$ and $\text{\hspace{0.17em}}\sqrt{1+\mathrm{cos}\text{\hspace{0.17em}}x},$ respectively.

For the half-angle formula given in the previous exercise for $\text{\hspace{0.17em}}\mathrm{tan}\left(\frac{x}{2}\right),$ explain why dividing by 0 is not a concern. (Hint: examine the values of $\text{\hspace{0.17em}}\mathrm{cos}\text{\hspace{0.17em}}x\text{\hspace{0.17em}}$ necessary for the denominator to be 0.)

## Algebraic

For the following exercises, find the exact values of a) $\text{\hspace{0.17em}}\mathrm{sin}\left(2x\right),$ b) $\text{\hspace{0.17em}}\mathrm{cos}\left(2x\right),$ and c) $\text{\hspace{0.17em}}\mathrm{tan}\left(2x\right)\text{\hspace{0.17em}}$ without solving for $\text{\hspace{0.17em}}x.$

If $\text{\hspace{0.17em}}\mathrm{sin}\text{\hspace{0.17em}}x=\frac{1}{8},$ and $\text{\hspace{0.17em}}x\text{\hspace{0.17em}}$ is in quadrant I.

a) $\text{\hspace{0.17em}}\frac{3\sqrt{7}}{32}\text{\hspace{0.17em}}$ b) $\text{\hspace{0.17em}}\frac{31}{32}\text{\hspace{0.17em}}$ c) $\text{\hspace{0.17em}}\frac{3\sqrt{7}}{31}$

#### Questions & Answers

how can are find the domain and range of a relations
austin Reply
A cell phone company offers two plans for minutes. Plan A: $15 per month and$2 for every 300 texts. Plan B: $25 per month and$0.50 for every 100 texts. How many texts would you need to send per month for plan B to save you money?
Diddy Reply
6000
Robert
more than 6000
Robert
can I see the picture
Zairen Reply
How would you find if a radical function is one to one?
Peighton Reply
how to understand calculus?
Jenica Reply
with doing calculus
SLIMANE
Thanks po.
Jenica
Hey I am new to precalculus, and wanted clarification please on what sine is as I am floored by the terms in this app? I don't mean to sound stupid but I have only completed up to college algebra.
rachel Reply
I don't know if you are looking for a deeper answer or not, but the sine of an angle in a right triangle is the length of the opposite side to the angle in question divided by the length of the hypotenuse of said triangle.
Marco
can you give me sir tips to quickly understand precalculus. Im new too in that topic. Thanks
Jenica
if you remember sine, cosine, and tangent from geometry, all the relationships are the same but they use x y and r instead (x is adjacent, y is opposite, and r is hypotenuse).
Natalie
it is better to use unit circle than triangle .triangle is only used for acute angles but you can begin with. Download any application named"unit circle" you find in it all you need. unit circle is a circle centred at origine (0;0) with radius r= 1.
SLIMANE
What is domain
johnphilip
the standard equation of the ellipse that has vertices (0,-4)&(0,4) and foci (0, -15)&(0,15) it's standard equation is x^2 + y^2/16 =1 tell my why is it only x^2? why is there no a^2?
Reena Reply
what is foci?
Reena Reply
This term is plural for a focus, it is used for conic sections. For more detail or other math questions. I recommend researching on "Khan academy" or watching "The Organic Chemistry Tutor" YouTube channel.
Chris
how to determine the vertex,focus,directrix and axis of symmetry of the parabola by equations
Bryssen Reply
i want to sure my answer of the exercise
meena Reply
what is the diameter of(x-2)²+(y-3)²=25
Den Reply
how to solve the Identity ?
Barcenas Reply
what type of identity
Jeffrey
Confunction Identity
Barcenas
how to solve the sums
meena
hello guys
meena
For each year t, the population of a forest of trees is represented by the function A(t) = 117(1.029)t. In a neighboring forest, the population of the same type of tree is represented by the function B(t) = 86(1.025)t.
Shakeena Reply
by how many trees did forest "A" have a greater number?
Shakeena
32.243
Kenard
how solve standard form of polar
Rhudy Reply
what is a complex number used for?
Drew Reply
It's just like any other number. The important thing to know is that they exist and can be used in computations like any number.
Steve
I would like to add that they are used in AC signal analysis for one thing
Scott
Good call Scott. Also radar signals I believe.
Steve
They are used in any profession where the phase of a waveform has to be accounted for in the calculations. Imagine two electrical signals in a wire that are out of phase by 90°. At some times they will interfere constructively, others destructively. Complex numbers simplify those equations
Tim

### Read also:

#### Get the best Precalculus course in your pocket!

Source:  OpenStax, Precalculus. OpenStax CNX. Jan 19, 2016 Download for free at https://legacy.cnx.org/content/col11667/1.6
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Precalculus' conversation and receive update notifications?

 By By