# Maths

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

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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 180°, 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

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👨‍🏫
Reiny

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