Siow Lin Hoe's answer to Vignesh anand's Secondary 3 A Maths Singapore question.
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Date Posted:
3 years ago
Can explain after cos 2x=-cos40. How u got 2x = 140?
“-“ means it should be at second and third quadrant.
Cos40 is the value you will get.
Meaning which you angle must be located at 180-40=140.
Example
If a question ask:
Cos 40 then it is straight forward cos 40
If
Cos140 then you will get -cos40.
Same for sin230 is equal to -sin50.
Cos40 is the value you will get.
Meaning which you angle must be located at 180-40=140.
Example
If a question ask:
Cos 40 then it is straight forward cos 40
If
Cos140 then you will get -cos40.
Same for sin230 is equal to -sin50.
That I understand. My qn is cos 2x= -cos 40.
Next step there is only 2x. Where the cos went in cos 2x?
Next step there is only 2x. Where the cos went in cos 2x?
If cos2x = -cos40 = cos 140 = cos -140 = cos 220 = cos -220.
Then your 2x is equal to 140, -140, 220, -220.
Example if sin x = sin 60 = sin 120
x=?
x = 60, 120.
Then your 2x is equal to 140, -140, 220, -220.
Example if sin x = sin 60 = sin 120
x=?
x = 60, 120.
Oh ok. I got it. Thanks a lot
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