A geometry problem by Mehul Chaturvedi

Geometry Level 3

c o s ( a ) + c o s ( 3 a ) + c o s ( 5 a ) . . . . . . . . + c o s ( ( 2 n − 1 ) a ) = ? cos(a)+cos(3a)+cos(5a)........ + cos((2n-1)a) = ?

AN IIT JEE Question(Not original)

3 4 s i n ( a ) c o s e c ( a ) \frac { 3 }{ 4 } sin(a)cosec(a) 3 s i n ( n a ) s e c ( a ) 3sin(na)sec(a) 1 2 s i n ( 2 n a ) c o s e c ( a ) \frac { 1 }{ 2 } sin(2na)cosec(a) 2 s i n ( n a ) c o s e c ( a ) 2sin(na)cosec(a)

Mehul Chaturvedi by Mehul Chaturvedi , 22, India

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2 solutions

I limit n to only 3, and i assumed a value for a, then tried all the choices and only one matched.. :3

0 Helpful 0 Interesting 0 Brilliant 0 Confused
Ramesh Goenka
Oct 10, 2014

put n as 1 ... only one option matches .. which is our answer.. :)

0 Helpful 0 Interesting 0 Brilliant 0 Confused

hey find other method than hit and try

Mehul Chaturvedi - 6 years, 8 months ago

and if there would be no option

Mehul Chaturvedi - 6 years, 8 months ago

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