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adnanchowdhury

  • 4 years ago

Trigonometric Identities question: http://d.pr/i/FsnP I got my answer to be 3/2 but the mark scheme says -3/2.

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  1. .Sam.
    • 4 years ago
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    \[3\cos2 \beta+19\cos \beta+13=0\] \[Use ~~~~\cos2\beta=2\cos^2\beta-1\] \[3(2\cos^2\beta-1)+19\cos \beta+13=0\] \[6\cos^2\beta-3+19\cos \beta+13=0\] \[6\cos^2\beta+19\cos \beta+10=0\] \[(2\cos \beta+5)(3\cos \beta+2)=0\] \[\cos \beta=-5/2, \cos \beta =-2/3\]

  2. adnanchowdhury
    • 4 years ago
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    Thanks. I made a silly mistake.

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