Given that sinA=2/5 and that A is obtuse,find the value of cosA.

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asked by Raj
yesterday at 4:18am
sinA = 2/5, so
cosA = √21/5
But A is in QII, so cosA = -√21/5

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posted by oobleck
yesterday at 6:51am
Why not sqrt(5^2-2^2)/5 ?

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  1. oobleck gave you the correct answer of cosA = -√21/5

    You asked, Why not sqrt(5^2-2^2)/5 ?
    well, isn't sqrt(5^2-2^2)/5 = √21/5 ?

    The reason cosA is negative???
    you said angle A is obtuse, so it must be between 90 and 180Β°, that is,
    it must be an angle in the 2nd quadrant.
    You should know the CASE rule, and according to that the cosine of a
    2nd quad angle is negative

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