Trigonometry – Problems with Right-Angled Triangles

Students are guided to discover the Sine, Cosine and Tangent ratios of right-angled triangles.  As learning progresses they learn how to calculate a missing angle and length in right-angled triangles.

This topic takes place in Year 11 Term 3 and follows on from Pythagoras’ Theorem.

Trigonometry – Problems with Right-Angled Triangles Lessons
Revision Lessons

Prerequisite Knowledge

  • express a multiplicative relationship between two quantities
  • understand and use proportion as equality of ratios
  • know the formulae for: Pythagoras’ theorem, a2 + b2 = c2
  • apply angle facts, triangle congruence, similarity and properties of quadrilaterals to conjecture and derive results about angles and sides, including Pythagoras’ Theorem and the fact that the
  • base angles of an isosceles triangle are equal, and use known results to obtain simple proofs

Success Criteria

  • know the trigonometric ratios, Sin ϑ = Opp/Hyp, Cos ϑ = Adj/Hyp, Tan ϑ = Opp/Adj.
  • apply them to find angles and lengths in right-angled triangles and, where possible, general triangles in two dimensional figures.
  • know the exact values of Sin ϑ and Cos Sin ϑ for ϑ = 0°, 30°, 45°, 60°, and 90°.; know the exact value of Tan ϑ for 0°, 30°, 45° and 60°.

Key Concepts

  • Sin, Cos and Tan are trigonometric functions that are used to find lengths and angles in right-angled triangles.
  • The ‘hypotenuse’ is opposite the right angle, the ‘opposite’ refers to the side that is opposite the angle in question and ‘adjacent’ side runs adjacent to the angle.
  • The inverse operations of sin, cos and tan are pronounced arcos, arcsin and arctan.

Common Misconceptions

  • Students often have difficulty knowing which trigonometric ratio to apply. Encourage them to clearly label the sides to identify the correct ratio.
  • Use SOHCAHTOA as a memory aid as students often forget the trigonometric ratios.
  • When using trigonometric ratios to calculate angles students often forget to use the inverse functions.
  • Students often try to apply right-angled formulae to non-right-angled triangles.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes:

<a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Mr Mathematics Blog

Deep Dive Mathematics Inspection

My experience as a Head of Mathematics going through the deep dive OFSTED inspection.

Calculating a Reverse Percentage

How to use ratios and proportional reasoning as a model for finding the original amount after a percentage change.

Area of Compound Shapes

To find the area of compound shapes students need to understand what the word compound means.