trig prob (1 Viewer)

[Timothy.Siu]

cos² x=2cos² (x/2) from 0 to 360 degrees

 

[QuLiT]

cos^2x = 2cos^2(x/2)
-cos^2x = -2cos^2(x/2)
1-cos^2x= 1-2cos^2(x/2)
1-cos^2x=cosx
cos^2x + cosx -1=0

cosx = -1/2plus or minus sqrt(5)/2

try that and see if you get the answer=S

 

[JasonNg1025]

You can't solve that as a normal quadratic because one is 2x and one is x.
Also, 1 - 2 cos²(x/2) does not equal cos x. It goes the other way around.
Also, the question L.H.S. is cos²x. Makes it a lot simpler

But the method is similar:
cos²x = 2 cos² (x/2)
cos²x - 1 = 2 cos² (x/2) - 1
cos²x - 1 = cos x
cos²x - cos x - 1 = 0

^^Solve that quadratic, and it should be ok

 

[QuLiT]

woops got double angle the wrong way round=.=

 

[Timothy.Siu]

thanks

 

[tommykins]

回复: Re: trig prob

You can't solve that as a normal quadratic because one is 2x and one is x.
Also, 1 - 2 cos²(x/2) does not equal cos x. It goes the other way around.
Also, the question L.H.S. is cos²x. Makes it a lot simpler

But the method is similar:
cos²x = 2 cos² (x/2)
cos²x - 1 = 2 cos² (x/2) - 1
cos²x - 1 = cos x
cos²x - cos x - 1 = 0

^^Solve that quadratic, and it should be ok

Umm did you accelerate 4unit math in yr 9?

 

[shaon0]

You can't solve that as a normal quadratic because one is 2x and one is x.
Also, 1 - 2 cos²(x/2) does not equal cos x. It goes the other way around.
Also, the question L.H.S. is cos²x. Makes it a lot simpler

But the method is similar:
cos²x = 2 cos² (x/2)
cos²x - 1 = 2 cos² (x/2) - 1
cos²x - 1 = cos x
cos²x - cos x - 1 = 0

^^Solve that quadratic, and it should be ok

What school do you go to?

 

[midifile]

What school do you go to?

Toongabbie Christian School according to his profile

 

[shaon0]

Toongabbie Christian School according to his profile

Okay. I think he is set for a 100 UAI with english as his last subject to do in Year 12.