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Verify the identity: tan ( π θ ) = tan θ .

tan ( π θ ) = tan ( π ) tan θ 1 + tan ( π ) tan θ                  = 0 tan θ 1 + 0 tan θ                  = tan θ
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Using sum and difference formulas to solve an application problem

Let L 1 and L 2 denote two non-vertical intersecting lines, and let θ denote the acute angle between L 1 and L 2 . See [link] . Show that

tan θ = m 2 m 1 1 + m 1 m 2

where m 1 and m 2 are the slopes of L 1 and L 2 respectively. ( Hint: Use the fact that tan θ 1 = m 1 and tan θ 2 = m 2 . )

Diagram of two non-vertical intersecting lines L1 and L2 also intersecting the x-axis. The acute angle formed by the intersection of L1 and L2 is theta. The acute angle formed by L2 and the x-axis is theta 1, and the acute angle formed by the x-axis and L1 is theta 2.

Using the difference formula for tangent, this problem does not seem as daunting as it might.

tan θ = tan ( θ 2 θ 1 )         = tan θ 2 tan θ 1 1 + tan θ 1 tan θ 2         = m 2 m 1 1 + m 1 m 2
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Investigating a guy-wire problem

For a climbing wall, a guy-wire R is attached 47 feet high on a vertical pole. Added support is provided by another guy-wire S attached 40 feet above ground on the same pole. If the wires are attached to the ground 50 feet from the pole, find the angle α between the wires. See [link] .

Two right triangles. Both share the same base, 50 feet. The first has a height of 40 ft and hypotenuse S. The second has height 47 ft and hypotenuse R. The height sides of the triangles are overlapping. There is a B degree angle between R and the base, and an a degree angle between the two hypotenuses within the B degree angle.

Let’s first summarize the information we can gather from the diagram. As only the sides adjacent to the right angle are known, we can use the tangent function. Notice that tan β = 47 50 , and tan ( β α ) = 40 50 = 4 5 . We can then use difference formula for tangent.

tan ( β α ) = tan β tan α 1 + tan β tan α

Now, substituting the values we know into the formula, we have

                     4 5 = 47 50 tan α 1 + 47 50 tan α 4 ( 1 + 47 50 tan α ) = 5 ( 47 50 tan α )

Use the distributive property, and then simplify the functions.

4 ( 1 ) + 4 ( 47 50 ) tan α = 5 ( 47 50 ) 5 tan α 4 + 3.76 tan α = 4.7 5 tan α 5 tan α + 3.76 tan α = 0.7 8.76 tan α = 0.7 tan α 0.07991 tan 1 ( 0.07991 ) .079741

Now we can calculate the angle in degrees.

α 0.079741 ( 180 π ) 4.57
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Access these online resources for additional instruction and practice with sum and difference identities.

Key equations

Sum Formula for Cosine cos ( α + β ) = cos α cos β sin α sin β
Difference Formula for Cosine cos ( α β ) = cos α cos β + sin α sin β
Sum Formula for Sine sin ( α + β ) = sin α cos β + cos α sin β
Difference Formula for Sine sin ( α β ) = sin α cos β cos α sin β
Sum Formula for Tangent tan ( α + β ) = tan α + tan β 1 tan α tan β
Difference Formula for Tangent tan ( α β ) = tan α tan β 1 + tan α tan β
Cofunction identities sin θ = cos ( π 2 θ ) cos θ = sin ( π 2 θ ) tan θ = cot ( π 2 θ ) cot θ = tan ( π 2 θ ) sec θ = csc ( π 2 θ ) csc θ = sec ( π 2 θ )

Key concepts

  • The sum formula for cosines states that the cosine of the sum of two angles equals the product of the cosines of the angles minus the product of the sines of the angles. The difference formula for cosines states that the cosine of the difference of two angles equals the product of the cosines of the angles plus the product of the sines of the angles.
  • The sum and difference formulas can be used to find the exact values of the sine, cosine, or tangent of an angle. See [link] and [link] .
  • The sum formula for sines states that the sine of the sum of two angles equals the product of the sine of the first angle and cosine of the second angle plus the product of the cosine of the first angle and the sine of the second angle. The difference formula for sines states that the sine of the difference of two angles equals the product of the sine of the first angle and cosine of the second angle minus the product of the cosine of the first angle and the sine of the second angle. See [link] .
  • The sum and difference formulas for sine and cosine can also be used for inverse trigonometric functions. See [link] .
  • The sum formula for tangent states that the tangent of the sum of two angles equals the sum of the tangents of the angles divided by 1 minus the product of the tangents of the angles. The difference formula for tangent states that the tangent of the difference of two angles equals the difference of the tangents of the angles divided by 1 plus the product of the tangents of the angles. See [link] .
  • The Pythagorean Theorem along with the sum and difference formulas can be used to find multiple sums and differences of angles. See [link] .
  • The cofunction identities apply to complementary angles and pairs of reciprocal functions. See [link] .
  • Sum and difference formulas are useful in verifying identities. See [link] and [link] .
  • Application problems are often easier to solve by using sum and difference formulas. See [link] and [link] .

Questions & Answers

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
Is there any rule we can use to get the nth term ?
Anwar Reply
how do you get the (1.4427)^t in the carp problem?
Gabrielle Reply

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Source:  OpenStax, Precalculus. OpenStax CNX. Jan 19, 2016 Download for free at https://legacy.cnx.org/content/col11667/1.6
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