SOLUTION: Please help me with this question,If SinA=2/3,and A is obtuse, find the exact value of, 1)CosA, 2)sin2A, 3) tan2A

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Question 1190009: Please help me with this question,If SinA=2/3,and A is obtuse, find the exact value of,
1)CosA, 2)sin2A, 3) tan2A
Found 2 solutions by MathLover1, ikleyn:
Answer by MathLover1(20850)   (Show Source): You can put this solution on YOUR website!

given:
and is obtuse
if than

° (obtuse)

if
=> => ,
than


1)


2)







3)









Answer by ikleyn(52797)   (Show Source): You can put this solution on YOUR website!
.
Please help me with this question, If SinA=2/3,and A is obtuse, find the exact value of,
1)cosA, 2)sin2A, 3) tan2A
~~~~~~~~~~~~~~~~


            The solution by @MathLover1 is  INCORRECT.

            I came to bring a correct solution. 


(1)  cos(A) =  =  =  =  = .


     Since A is obtuse, angle A is in QII, where cosine is negative, so we use sign  " - "  at the square root.


     It is first difference from the solution by @MathLover1.




(2)  sin(2A) = 2*sin(A)*cos(A) =  = .


     Notice sign  " - " in my answer.  It is the second difference from the solution by @MathLover1.




(3)  tan(2A) =  =  =   =  =  = .


     Notice sign  " - " in my answer.  It is the third difference from the solution by @MathLover1.

Solved     (correctly).



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