Trigonometry - EduqasCalculating an angle

The three trigonometric ratios; sine, cosine and tangent are used to calculate angles and lengths in right-angled triangles. The sine and cosine rules calculate lengths and angles in any triangle.

Part ofMathsGeometry and measure

Calculating an angle

The three trigonometric ratios can be used to calculate the size of an angle in a right-angled triangle.

Example

Calculate the angle QPR. Give the answer to one decimal place.

Triangle (PQR) with unknown angle x

Label the sides of the triangle \(o\), \(a\) and \(h\).

Next choose the correct ratio from \(s^o_h~c^a_h~t^o_a\).

In the triangle the lengths \(o\) and \(a\) are known.

Use: \(\tan{x} = \frac{o}{a}\)

\(\tan{x} = \frac{5}{7}\)

To calculate the angle use the inverse tan button on the calculator (\(\tan^{-1}\))

\(x = \tan^{-1} (\frac{5}{7})\)

\(x = 35.5^\circ\)

Question

Calculate the angle KJL. Give the answer to one decimal place.

Triangle (JKL) with unknown angle x