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Trigonometry

Browse High School Trigonometry
Stars indicate particularly interesting answers or
good places to begin browsing.
Selected answers to common questions:
DeMoivre's theorem.
Volume of a tank.
 Premature Simplification [08/01/2003]

Solve: 2tan x + sin^2 x * sec x = 1 + sec x
 Product Sum and Difference Formulae [11/20/1997]

(cos6theta+6cos4theta+15cos2theta+10)/(cos5theta+5cos3theta+10cos theta) = 2
cos theta.
 Projectile Dynamics [10/21/2000]

A particle is projected with initial velocity v and angle theta in a
parabolic path. How can I show that at time t, when the angle to the
horizontal is gamma, tan(gamma) = tan(theta)  (gt/v) sec(theta)?
 Proof by First Principles [11/16/2001]

How can I prove by first principles that the derivative of tan(x) is
sec^2(x)?
 Proof of a Trig Identity [03/13/1998]

Proving that (1 + sinX + cosX)/(1 + sinX  cosX) = (1 + cosX)/sinX.
 Proof of DeMoivre's Theorem [05/01/1997]

A typical induction proof: DeMoivre's theorem.
 Proof of the Parallelogram Law [08/07/1999]

How do you prove the parallelogram law geometrically, without using
vectors?
 Proof of Trig Identity [7/24/1996]

Prove (sin 2t + sin t)/(cos 2t + cos t + 1) = tan t is an identity.
 Proving a Trigonometric Identity [06/06/1999]

How can I prove the trigonometric identities cos(a+b)*cos(ab) = cos^2a 
sin^2b, and tan(x/2) = (1cosx)/(sinx)?
 Proving a Trigonometry Identity [05/19/2003]

Prove the following identity: cos 3x = 4 cos^3x  3cosx.
 Proving De Moivre's Theorem [12/03/1997]

Prove De Moivres theorem:  (cos(x)+isin(x))^n = cos(nx) + isin(nx) .
 Proving Identity for Sum of Cosines [10/17/2003]

How do you prove the identity for the sum of cosines?
 Proving Laws of Sines, Cosines [10/01/2003]

How do you prove the laws of sines and cosines?
 Proving Sine Identities [05/16/1998]

Can you help me prove the trigonometric identities for sin(A+B), sin(2A),
and sin(A/2)?
 Proving Trig Identities [11/15/1998]

How do I prove trig identities like sin^6x+cos^6x = 13sin^2xcos^2x?
 Proving Trig Identities vs. Solving Trig Equations [10/08/2006]

Proving the identity sinx/(1cosx)  (1+cosx)/sinx = 0 is done a
little differently than solving that same equation for x, though the
thinking is very similar. What is the key to proving the identity?
 Proving Trigonometric Identities [02/12/1999]

Tips on proving trigonometric identities, as well as a good general
procedure to use, worked out on cos^4(x)  sin^4(x) = cos(2*x).
 Proving Trigonometry Identities [01/07/1997]

Prove these trigonometry identities (tan x+cot x)tan x = sec^2 x and
(1+cos x)(1cos x) = sin^2 x
 Pythagorean and Other Triangle Theorems [1/6/1995]

Could you please show me the proof for the Pythagorean Theorem and list
the other triangle theorems that are based on it?
 The Pythagorean Identity for Cosine [11/11/1995]

If Sin 9 = x , what is the solution and result of Cos9.cos18.cos36?
 Pythagorean Identity of Trigonometry [02/16/1998]

Proof of the trigonometry identity sin^2 + cos^2 = 1.
 Radian Conversion [07/24/2012]

Used to performing trigonometry in degrees, a student gets thrown for a loop by
radians. Doctor Ian clarifies with an example based on an analogy of converting between
imperial and metric units of distance.
 Radians [03/22/2001]

I have searched everywhere for the history behind radians. Why did they
replace degrees?
 Radian to Degree Conversions [11/11/2001]

Can you give me a table of radian measures and their reference degree
measures for the unit circle?
 Radius of Circumscribed Circle [05/11/2001]

Where can I find a derivation of R = abc/4K?
 Rail Bend in Hot Weather [10/13/2002]

A 20ft piece of rail expands 1 in. in length during a hot spell. If
there are no expansion gaps, how high off the ground will the rail
rise?
 Raised Cosines [01/28/2001]

What are raised cosines, and how do they work?
 Ratios, Geometry, Trigonometry [06/10/1999]

A homeschool teacher asks for help with triangles, flagpoles, and
circles.
 Rearranging x=tan()sin() [05/01/2002]

How would I approach rearranging x = tan(y)*sin(y) into y = some
function of x?
 Reflections in Parabolic Mirrors [05/14/2005]

I know that in a parbola, any ray that starts at the focus and hits
the parabola is reflected parallel to the central axis of the
parabola. Can you explain or prove why that happens?
 Related Rates  Clock Hands [04/05/1999]

How fast is the distance between the tips of the hands on a watch
changing at one o'clock?
 Remembering Trig Functions [09/27/2001]

We have to memorize the sine, cosine, tangent, secant, cosecant, and
cotangent for 30, 45, and 60 degrees for precalculus class. Do you have
any tips on how to remember these?
 Replace Sin x with 1/n [12/19/2001]

Solve: (5 + 2sqrt(6))^(sin x) + (5  2sqrt(6))^(sin x) = 2sqrt(3).
 Right Triangle Kite Problem [01/16/1997]

When flying a kite, 150 ft. makes an an angle of 51 degrees with the
ground. Assume the string is straight; how is the kite?
 Right Triangle Sides [09/14/1998]

To use SOHCOHTOA, I need help finding which sides are opposite and
adjacent.
 Roots of a Complex Equation [04/28/1997]

Given that cos t + isin t is the root of z^3 + az + b = 0 in which sin t
is not equal to 0, and that a and b are real and not equal to zero, show
that a + b^2 = 1.
 Rotating a Point around an Angle [05/16/1997]

How do you derive the formulas for rotating a point around an angle?
 Scaling a Right Triangle [12/01/2003]

Start with a right triangle of known dimensions. Move the hypotenuse
by some distance towards the right angle. What are the dimensions of
the new triangle?
 Secant, Cosecant, and Cotangent [01/16/1997]

How do you understand the secant, cosecant, and cotangent functions?
 Shadow Moving Along a Wall [08/16/1999]

There is a light at the front of a circular arena. A boy starting from B
runs at the rate of 3 m/s toward the center C. At what rate will his
shadow be moving along the rear wall when he is halfway from B to C?
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