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trigonometry

by romitvsingh » Wed Jan 11, 2012 8:47 pm
the sin of angle B is 28/35. the angle measure is therefore sin -1( 28/35 ); angle B is approx 53.1 degree

My question is how do we get the value of angle B as 53.1 degree ?

pls explain in detail
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by avik.ch » Wed Jan 11, 2012 10:04 pm
You will never be asked to find the exact values of sin -1( 28/35 ).

What you can be asked is : Sin B = 28/35 then what is cos B or tan B. This seems logical.

Anyway, there is no way to find the exact value of 53.1 degree, unless you are provided with a trigonometric chart or a calculator.


Thanks !!
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by rijul007 » Thu Jan 12, 2012 12:59 am
As avik.ch said
GMAT would never ask a question where you would need to find the value of sine inverse


But if you were given options such as these:
A. 17.4
B. 34.8
C. 43.9
D. 53.1
E. 69.3


Heres how I would have done it:
Sin(0) = 0
As we increase the angle, sin also increases until sin(90) = 1


sin(x) = 28/35 = 4/5 = 0.8
sin(45) = 1/sqrt(2) = 0.7
sin(45) < sin(x)
45<x
Option A, B, and C are eliminated

sin(60) = sqrt(3)/2 = 0.86
sin(60) > sin(x)
60 > x
Option E also eliminated

Option D is correct



If two options were given between 45 and 60, we cannot find the ans and neither does GMAT expect us to.
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by GMATGuruNY » Thu Jan 12, 2012 1:19 am
Trigonometry is not tested on the GMAT. Any problem that discusses the sine, cosine or tangent of an angle -- or requires knowledge of these properties -- is not a legitimate GMAT question.
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by avik.ch » Thu Jan 12, 2012 1:40 am
The law has already been stated by Mitch sir.

That is the reason why OG never uses any trigonometry to explain any problems.
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