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India
Class XI

Evaluate trigonometric functions values

Lesson

When we evaluate a trigonometric function, such as $f(x)=\sin(x)$f(x)=sin(x), we can relate this process back to finding the ratio of the sides of a right triangle. For example, the following right triangle has side lengths of $3$3 units, $4$4 units, and $5$5 units.

We define the sine of the angle $\theta$θ as being the ratio of the side of the triangle opposite the angle and the hypotenuse of the triangle. In this case, the sine of the angle $\theta$θ is written as $\sin\theta=\frac{3}{5}$sinθ=35.

In a similar way, we can define the cosine of $\theta$θ, and the tangent of $\theta$θ. These three important trigonometric ratios are summarised below.

Trigonometric ratios

Sine: $\sin\theta=\frac{\text{Opposite }}{\text{Hypotenuse }}$sinθ=Opposite Hypotenuse

Cosine: $\cos\theta=\frac{\text{Adjacent }}{\text{Hypotenuse }}$cosθ=Adjacent Hypotenuse

Tangent: $\tan\theta=\frac{\text{Opposite }}{\text{Adjacent }}$tanθ=Opposite Adjacent

 

We can always use a calculator to find the value of $f(x)=\sin(x)$f(x)=sin(x), for any value of $x$x. In general the result will be a number with many decimal places. However there are some specific values of $x$x that result in function values that can be expressed in terms of surds, rational numbers, or even integers.

 

Exact value triangles

Below is a right isosceles triangle, with the equal sides of $1$1 unit. Using Pythagoras' theorem, we can see that the hypotenuse is $\sqrt{2}$2 units. Further, because the triangle has an interior angle sum of $\pi$π radians, and the base angles in an isosceles triangle are equal, we can deduce that the other two unknown angles are both $\frac{\pi}{4}$π4 radians.

Using trigonometric ratios listed above, we can see that:

  • $\sin\left(\frac{\pi}{4}\right)=\frac{1}{\sqrt{2}}=\frac{\sqrt{2}}{2}$sin(π4)=12=22
  • $\cos\left(\frac{\pi}{4}\right)=\frac{1}{\sqrt{2}}=\frac{\sqrt{2}}{2}$cos(π4)=12=22
  • $\tan\left(\frac{\pi}{4}\right)=\frac{1}{1}=1$tan(π4)=11=1

Notice in particular that $\sin\left(\frac{\pi}{4}\right)=\cos\left(\frac{\pi}{4}\right)$sin(π4)=cos(π4).

Now let's start with an equilateral triangle that has side lengths of $2$2 units. Remember all the angles in an equilateral triangle are $\frac{\pi}{3}$π3 radians.

By drawing a line that cuts the triangle into two congruent pieces, the base of each smaller triangle is now $1$1 unit, and the length of the centre line is $\sqrt{3}$3 units, using Pythagoras' theorem.

Here is one of the smaller triangles.

In this case the trigonometric ratios tell us that:

  • $\sin\left(\frac{\pi}{3}\right)=\frac{\sqrt{3}}{2}$sin(π3)=32
  • $\cos\left(\frac{\pi}{3}\right)=\frac{1}{2}$cos(π3)=12
  • $\tan\left(\frac{\pi}{3}\right)=\frac{\sqrt{3}}{1}=\sqrt{3}$tan(π3)=31=3
  • $\sin\left(\frac{\pi}{6}\right)=\frac{1}{2}$sin(π6)=12
  • $\cos\left(\frac{\pi}{6}\right)=\frac{\sqrt{3}}{2}$cos(π6)=32
  • $\tan\left(\frac{\pi}{6}\right)=\frac{1}{\sqrt{3}}=\frac{\sqrt{3}}{3}$tan(π6)=13=33

Notice here that $\sin\left(\frac{\pi}{3}\right)=\cos\left(\frac{\pi}{6}\right)$sin(π3)=cos(π6) and $\sin\left(\frac{\pi}{6}\right)=\cos\left(\frac{\pi}{3}\right)$sin(π6)=cos(π3).

 

Practice questions

QUESTION 1

The function $f\left(x\right)$f(x) is defined as $f\left(x\right)=\cos x$f(x)=cosx.

Find the exact value of $f\left(\frac{\pi}{6}\right)$f(π6).

Question 2

The function $f\left(x\right)$f(x) is defined as $f\left(x\right)=\sin x$f(x)=sinx.

Find the exact value of $f\left(-\frac{11\pi}{6}\right)$f(11π6).

Question 3

The function $f\left(x\right)$f(x) is defined as $f\left(x\right)=2\tan x+16$f(x)=2tanx+16.

Find the exact value of $f\left(\frac{\pi}{6}\right)$f(π6).

Outcomes

11.SF.TF.1

Positive and negative angles. Measuring angles in radians and in degrees and conversion from one measure to another. Definition of trigonometric functions with the help of unit circle. Truth of the identity sin^2 x + cos^2 x = 1, for all x. Signs of trigonometric functions and sketch of their graphs. Expressing sin (x + y) and cos (x + y) in terms of sin x, sin y, cos x and cos y. Deducing the identities like following: cot(x + or - y), sin x + sin y, cos x + cos y, sin x - sin y, cos x - cos y (see syllabus document)

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