Find the unknown parts of an oblique triangle with law of cosines

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a=21.3 

b=13.6

c=9.7

May 17th, 2015

Law of cosines  =


c2 = a2 + b2 − 2ab cos θ
9.7^2 = 21.3^2+13.6^2- 21.3 *13.6*cos θ

94.09    = 638.65 -289.68 cos θ

(94.09-638)/ (-289.68)  =cos θ

1.877 =cos θ

 θ = cos^-1( 1.877) = 

values of cos θ should be between 0 to 1

but here coming more than 1 so plz check the dimension once ie  check once the values of a , b ,c 

any doubt plz let me know 

I will be happy to help you 


May 17th, 2015

Sry I did the wrong calculation

I am giving the correct solution within 5 mints

May 17th, 2015

9.7^2 = 21.3^2+13.6^2- 2 *21.3 *13.6*cos θ

   94.09  =  638.65 -579.36 cos θ

Cos θ = (638.65-94.09)/ 579.36 = 0.9399

θ  = Cos^-1(0.9399)  = 19.65  

May 17th, 2015

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