##### Equations of Form trig\((u)=C\) where "trig" is One the Six Standard Trigonometric Functions and \(C\) is a real number

Regardless of the initial form of a trigonometric equation, the solver eventually has to solve one or more equations in the form stated above, so we need to get a good grasp on that type of equation before moving on to other types of equations. The best method of explanation in this section is to show several examples. Throughout this section I will be stating solutions in terms of radians. If you don't have a solid memory of points on the unit circle, you probably want to have a printed copy of the unit circle while reading and working through this section.

###### Example14.10.1

Determine all solutions to the equation \(\sin(t)=-\frac{\sqrt{3}}{2}\text{.}\)

Given the definition of the sine function, this equation is analogous to the following question: What are all of the points on the unit circle with a \(y\)-coordinate of \(-\frac{\sqrt{3}}{2}\text{.}\)

Over the interval \([0,2\pi)\text{,}\) there are two points on the unit circle with a \(y\)-coordinate of \(-\frac{\sqrt{3}}{2}\text{.}\) The points correspond to \(t=\frac{4\pi}{3}\) and \(t=\frac{5\pi}{3}\text{.}\) So two solutions to the stated equation occur where \(t=\frac{4\pi}{3}\) and \(t=\frac{5\pi}{3}\text{.}\) But these are not the only solutions.

When drawn in standard position, any two angles whose radian measurements vary by any integer multiple of \(2\pi\) are coterminal and, consequently, their six basic trigonometric values are equal. As such, we can add any integer multiple of \(2\pi\) to either of our stated solutions and come up with a another solution to the equation. This leads to the following conclusion.

The solutions to the equation \(\sin(t)=-\frac{\sqrt{3}}{2}\) consist of all angles whose radian measurement can be written either in the form

where \(k\) can be any integer.

The above statement is called the general solution to the equation.

###### Example14.10.2

Determine the general solution to the equation \(\sec(2t)=2\text{.}\)

The secant function is the reciprocal of the cosine function, so the stated equation is equivalent to the equation \(\cos(2t)=\frac{1}{2}\text{.}\) The first thing we need to establish are the two angles over the interval \([0,2\pi)\) where the \(x\)-coordinate is \(\frac{1}{2}\text{.}\) Those two points occur at angles with radian measurement \(\frac{\pi}{3}\) and \(\frac{5\pi}{3}\text{.}\)

The period of the cosine function is \(2\pi\text{,}\) so any integer multiple of \(2\pi\) added to either of the stated values results in another point where the \(x\)-coordinate is \(\frac{1}{2}\text{.}\)

In the stated equation, the variable \(t\) is multiplied by \(2\) before the secant function is applied, so our solutions will come from the following equations (where \(k\) can be any integer).

Multiplying both sides of both equations by \(\frac{1}{2}\) gives us the following.

In conclusion, the general solution to the equation \(\sec(2t)=2\) is

where \(k\) can be any integer.

###### Example14.10.3

Determine all solutions to the equation \(\tan\left(\frac{t}{2}\right)=1\)

In terms of a point on the unit circle, the tangent value is determine by the ratio \(\frac{y}{x}\text{,}\) so solving the stated equation entails, in part, answering the following question: Where on the unit circle are the \(x\)-coordinate and the \(y\)-coordinate equal to one another? The two locations over the interval \([0,2\pi)\) where that is true occur at \(\frac{\pi}{4}\) and \(\frac{5\pi}{4}\text{.}\)

One way we could state the radian measurement all points where the \(x\) and \(y\) coordinates are equal is to say that they can be written either in the form \(\frac{\pi}{4}+2\pi k\) or in the form \(\frac{5\pi}{4}+2\pi k\) where \(k\) can be any integer. But because the period of the basic tangent function is \(\pi\text{,}\) we can generate all such points by simply stating \(\frac{\pi}{4}+\pi k\) where \(k\) can be any integer. As the value of \(k\) moves through the sequence \(0,\,1,\,2,\,3,\,...\text{,}\) this latter formula alternately gives us radian measurements equal to or coterminal with \(\frac{\pi}{4},\,\frac{5\pi}{4},\,\frac{\pi}{4},\,\frac{5\pi}{4},\,...\text{.}\)

Solutions to the stated equation will come from the following equation (where \(k\) can be any integer).

Multiplying both sides of the above equation by \(2\) results in the following.

In conclusion, any solution to the equation \(\tan\left(\frac{t}{2}\right)=1\) can be written in the form

where \(k\) can be any integer.

###### Example14.10.4

Determine all solutions to the equation \(\csc(3t)=\frac{2\sqrt{3}}{3}\) over the interval \([0,2\pi)\text{.}\)

Because the sine function and the cosecant function are reciprocals, the stated equation is equivalent to the equation \(\sin(3t)=\frac{3}{2\sqrt{3}}\text{,}\) so we are interested in points along the unit circle where the \(y\)-coordinate is \(\frac{3}{2\sqrt{3}}\text{.}\) That number is not familiar looking, so let's try rationalizing the denominator to see if something familiar comes about.

Ah, something familiar. There are two locations along the unit circle where the \(y\)-coordinate is \(\frac{\sqrt{3}}{2}\text{.}\) Those locations correspond to the radian measurements of \(\frac{\pi}{3}\) and \(\frac{2\pi}{3}\text{.}\) If the equation were simply \(\csc(t)=\frac{2\sqrt{3}}{3}\) those two values would be the solutions over the interval \([0,2\pi)\text{.}\) But, because of the factor of \(3\) in the stated equation, we have a bit of work still to do.

The general solution to the equation comes from the following two equations (where \(k\) can be any integer).

Multiplying both sides of both equations by \(\frac{1}{3}\) results in the following.

We are only looking for solutions that lie over the interval \([0,2\pi)\text{.}\) When you are looking for solutions to an equation of form trig\((ct)=\)number, where \(c\) is a positive integer, but you are only interested in solutions that lie over the interval \([0,2\pi)\text{,}\) you want to take your general solution and let \(k\) assume the values \(0,\,1,\,2,\,...,c-1\text{.}\) So in our case, we will let \(k\) take on the values \(0\text{,}\) \(1\text{,}\) and \(2\text{.}\)

In conclusion, the solutions to the equation \(\csc(3t)=\frac{2\sqrt{3}}{3}\) over the interval \([0,2\pi)\) are stated below.

###### Example14.10.5

Determine all solutions to the equation \(\cot(2t)=-\sqrt{3}\) over the interval \([0,2\pi)\text{.}\)

In terms of the unit circle, the cotangent value at a given point is equal to the ratio \(\frac{x}{y}\text{.}\) So in order for the cotangent function to have a value of \(-\sqrt{3}\text{,}\) the \(x\)-coordinate of the point must be \(-\sqrt{3}\) times the \(y\)-coordinate of the point. The two such points that are on the unit circle are \(\left(-\frac{\sqrt{3}}{2},\frac{1}{2}\right)\) and \(\left(\frac{\sqrt{3}}{2},-\frac{1}{2}\right)\text{.}\) Those two points correspond to the radian measurements of \(\frac{5\pi}{6}\) and \(\frac{11\pi}{6}\text{.}\)

The general solution to the stated equation can be written stated below (where \(k\) can be any integer).

As was the case with the tangent equation, we could simplify this to a single equation (because the period of the basic cotangent function is \(\pi\)). However, because we are only looking for solutions over the interval \([0,2\pi)\text{,}\) I find it easier to stick with the two equations and let \(k\) take on the values \(0\) and \(1\) (stopping at \(1\) because that is one less than the coefficient on \(t\) in the original equation, \(2\)). Before doing that, let's isolate \(t\) by multiplying both sides of the two equations by \(\frac{1}{2}\text{.}\) The result follows.

Let's go ahead and replace \(k\) with \(0\) and \(1\text{.}\)

In conclusion, the solutions to the equation \(\cot(2t)=-\sqrt{3}\) that lie on the interval \([0,2\pi)\) are stated below.

###### Example14.10.6

Determine all solutions to the equation \(\cos(4t)=-1\) over the interval \([0,2\pi)\text{.}\)

Along the unit circle, the cosine value at a given point is equal to the \(x\)-coordinate of the point. There is only one point on the unit circle that has an \(x\)-coordinate of \(-1\text{,}\) the point \((-1,0)\text{.}\) This point occurs at angles with a radian value of \(\pi\) or are coterminal with \(\pi\text{.}\) So the solutions to the stated equation correspond with the solutions to the following equation (where \(k\) can be any integer).

Dividing both sides of the last equation by \(4\) we get the following.

Because the coefficient on \(t\) in the original equation is \(4\text{,}\) we can determine all of the solutions to equation that fall over the interval \([0,2\pi)\) by replacing \(k\) with \(0\text{,}\) \(1\text{,}\) \(2\text{,}\) and \(3\text{.}\) Let's do that.

In conclusion, the solutions to the equation \(\cos(4t)=-1\) that occur on the interval \([0,2\pi)\) are stated below.