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Signs in the expansion

Exercise and reference answers

Questions
  1. Question
    cos(x + y) expands with which sign in the middle?
    Choices
    1. plus
    2. no sign
    3. minus
    Correct choice
    minus
  2. Question
    sin(x + y) expands with which sign?
    Choices
    1. plus
    2. minus
    3. no sign
    Correct choice
    plus
  3. Question
    cos(x − y) expands with which sign?
    Choices
    1. minus
    2. plus
    3. no sign
    Correct choice
    plus
  4. Question
    sin(x − y) expands with which sign?
    Choices
    1. plus
    2. no sign
    3. minus
    Correct choice
    minus

Explanation

The sign flips between the cosine and sine expansions, which is the usual slip. Source: syllabus Maths, Ch 3 Trigonometric Functions.
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