Thursday 5 October 2017

`sin(2x) - sin(x) = 0` Find the exact solutions of the equation in the interval [0, 2pi).

`sin(2x)-sin(x)=0 , 0<=x<=2pi`


using the identity `sin(2x)=2sin(x)cos(x)`


`sin(2x)-sin(x)=0`


`=2sin(x)cos(x)-sin(x)=0`


`=(sin(x))(2cos(x)-1)=0`


solving each part separately,


`sin(x)=0`  or `2cos(x)-1=0`


General solutions for sin(x)=0 are,


`x=0+2(pi)n, x=pi+2(pi)n`


solutions for the range `0<=x<=2pi`  are,


`x=0 , x=pi , x=2pi`


Now solving 2cos(x)-1=0,


`2cos(x)=1`


`cos(x)=1/2`


General solutions for cos(x)=1/2 are,


`x=(pi)/3+2pin , x=(5pi)/3+2pin`


solutions for the range `0<=x<=2pi` are,


`x=pi/3 , x=(5pi)/3`


Hence all the solutions are,


`x=0,pi,2pi,pi/3 , (5pi)/3`

`sin(2x)-sin(x)=0 , 0<=x<=2pi`


using the identity `sin(2x)=2sin(x)cos(x)`


`sin(2x)-sin(x)=0`


`=2sin(x)cos(x)-sin(x)=0`


`=(sin(x))(2cos(x)-1)=0`


solving each part separately,


`sin(x)=0`  or `2cos(x)-1=0`


General solutions for sin(x)=0 are,


`x=0+2(pi)n, x=pi+2(pi)n`


solutions for the range `0<=x<=2pi`  are,


`x=0 , x=pi , x=2pi`


Now solving 2cos(x)-1=0,


`2cos(x)=1`


`cos(x)=1/2`


General solutions for cos(x)=1/2 are,


`x=(pi)/3+2pin , x=(5pi)/3+2pin`


solutions for the range `0<=x<=2pi` are,


`x=pi/3 , x=(5pi)/3`


Hence all the solutions are,


`x=0,pi,2pi,pi/3 , (5pi)/3`

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