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When solving a trigonometric equation, what is the CAST diagram used for?
ATo find the size of the reference angle
BTo find the period of the function
CTo find where the ratio is positive
DTo find the value of the amplitude
Answer & Solution
Correct answer: C. To find where the ratio is positive
1. A calculator returns only one angle, called the reference angle, for a given ratio.
2. But the same ratio has the same sign in two of the four quadrants.
3. The CAST diagram records which trigonometric ratios are positive in each quadrant.
4. It is therefore used to decide which quadrants the solutions lie in.
5. The reference angle itself comes from the calculator, not from the diagram.
_Source: Siyavula Mathematics Grade 12 (Everything Maths, CC BY 4.0), Chapter 4: Trigonometry_
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