MYP · Mathematics (Standard) MYP Year 4
Trigonometry
MYP Year 4 Mathematics — Trigonometry (Measurement & Geometry strand)
26 min readStandardCore measurement/geometry strand — appears in most MYP5 unit tests and at least one Criterion A/C investigation involving real-world triangles
Everything in this topic is one idea repeated in five costumes: fix an angle, and the ratios of a triangle's sides are fixed too. Right-triangle trig (SOH CAH TOA) only works when there's a 90° corner to lean on. Real triangles — garden plots, roof trusses, ship routes — usually don't have one, so the sine and cosine rules extend the same idea to any triangle. Bearings, area-without-height, and the periodic graphs are just that one relationship applied to different pictures.
Overview — The Shape of the Chapter
Why you need more than SOH CAH TOA
- Right-triangle ratios (sin, cos, tan) only apply when one angle is exactly 90°. A surveyor's plot, a ship's course, or a hillside garden almost never hands you a right angle for free.
- The sine rule and cosine rule solve any triangle, provided you have enough independent measurements — three sides, or two sides plus an angle, or two angles plus a side.
- The area formula is the same trick again: it manufactures the 'height' you'd normally need to measure, using an angle instead.
- Bearings problems and trig graphs aren't separate topics — they're the sine/cosine rule and the right-triangle ratios wearing a compass or a wave shape.
The shape of the chapter
Command terms this topic actually tests
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Calculate | Produce a numerical answer with working shown | AO1/AO2 | Method mark for correct formula/substitution, separate mark for final value — a right answer with no working can still lose the method mark |
| Show that | Prove a given result step-by-step, not just state it | AO2 | You must display every algebraic step (e.g. cos90°=0 explicitly) — skipping to the answer scores 0 even if correct |
| Explain | Give a reason with clear logical link to the maths | AO3 | A bare assertion ('C got bigger so c got bigger') without referencing why (behaviour of cosC) scores 0/1 |
| Analyse | Break a situation into components and compare their effect | AO3/AO4 | Requires comparing sensitivities (e.g. which measurement error matters most) with actual numbers, not just opinion |
| Justify | Support a conclusion with mathematical evidence | AO3 | Needs a calculation or property cited, not just 'because it makes sense' |
| Identify / Describe | Name or state a feature briefly | AO1 | One correct, specific fact is enough — vague field-measurement waffle earns 0 |
Key point
Before touching a calculator, decide which rule fits the data you actually have: SAS/SSS → cosine rule; a complete angle-side pair (AAS/ASA/SSA) → sine rule. Choosing the wrong rule wastes the whole question even with perfect arithmetic.
Overview