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Grade: 12 

                        
Let α and β be non–zero real numbers such that 2(cos β – cos α ) + cos α cos β = 1. Then which of the following is/are true?

							
Let α and β be non–zero real numbers such that 2(cosβ – cosα) + cosα cosβ = 1. Then which of the following is/are true?
 
2 years ago

Answers : (1)

Umang Kumar
15 Points 
							
2(cosb-cosa)+cosa.cosb=1
2cosb-2cosa+cosa.cosb=1
cosb(2+cosa)-(2cosa+1)=0
cos\beta =\frac{2cos\alpha +1}{2+cos\alpha }
Now converting cos\beta =\frac{1-tan^{2}\frac{\beta }{2}}{1+tan^{2}\frac{\beta }{2}}
cos\alpha =\frac{1-tan^{2}\frac{\alpha }{2}}{1+tan^{2}\frac{\alpha }{2}}
now equating cosa and cosb in
cos\beta =\frac{2cos\alpha +1}{2+cos\alpha }
we get
\frac{1-tan^{2}\frac{\beta }{2}}{1+tan^{2}\frac{\beta }{2}}=\frac{3-tan^{2}\frac{\alpha }{2}}{3+tan^{2}\frac{\alpha }{2}}
After applying Componendo and dividendo method each side we get:
\frac{2}{-2tan^{2}\frac{\beta }{2}}=\frac{6}{-2tan^{2}\frac{\alpha }{2}}
After solving this we get:
\frac{1}{tan^{2}\frac{\beta }{2}}=\frac{3}{tan^{2}\frac{\alpha }{2}}
After cross multiplying we get:
tan^{2\frac{\alpha }{2}}=3tan^{2\frac{\beta }{2}}
Square root both side:
tan\frac{\alpha }{2}=\pm \sqrt{3}tan\frac{\beta }{2}
hence the answers are:-
1.tan\frac{\alpha }{2}+\sqrt{3}tan\frac{\beta }{2}=0
2.tan\frac{\alpha }{2}-\sqrt{3}tan\frac{\beta }{2}=0
 
2 years ago
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