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Homework Help: Sine, cosine

  1. Jan 25, 2012 #1
    1. The problem statement, all variables and given/known data


    2. Relevant equations

    3. The attempt at a solution

    I don't see how the textbook gets from step 1 to step 2. If anything, the cosines cancel and the answer should be (sine^2)x
  2. jcsd
  3. Jan 25, 2012 #2


    User Avatar
    Homework Helper

    One of the best known trigonometric identities is : [itex]\cos^2 x + \sin^2 x = 1[/itex]. To see this, just draw a right angle triangle, with one of the acute angles marked x and the hypotenuse measuring 1 unit. One side (opposite angle x) measures sin x and the other side (adjacent to angle x) measures cos x. You can immediately see the identity with Pythagoras' Theorem.

    Remember that [itex]\frac{a^2 + b^2}{a^2} \neq b^2[/itex]. Cancellation doesn't work that way. It is true, however that [itex]\frac{a^2b^2}{a^2} = b^2[/itex] (when a is nonzero).
  4. Jan 25, 2012 #3
    thanks, trig identities, I forgot about them
  5. Jan 25, 2012 #4

    You can't cancel a out unless you take the common factor out of parenthesis:


    Now you can cancel a's out and you will be left with 1-b/a

    Another example:

    (a-ab)/a= [a(1-b)]/a=1-b

    To cancel you always have to get the common factor out of parenthesis.
  6. Jan 25, 2012 #5


    Staff: Mentor

    You pretty much can't simplify trig expressions without having a few identities in mind, so I would advise you to spend some time reviewing them.

    I would also advise reviewing basic algebra, particularly fractions and rational expressions, since you seem to have forgotten those concepts, as well. You should wipe the word "cancel" from your mind, since students who are uncertain about what this actually means are prone to making mistakes.
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