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Solution 4.3:4d

From Förberedande kurs i matematik 1

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With the formula for double angles and the Pythagorean identity <math>\cos^2\!v + \sin^2\!v = 1</math>, we can express <math>\cos 2v</math> in terms of <math>\cos v</math>,
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<center> [[Image:4_3_4d.gif]] </center>
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{{Displayed math||<math>\begin{align}
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\cos 2v &= \cos^2\!v - \sin^2\!v\\[5pt]
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&= \cos^2\!v - (1-\cos^2\!v)\\[5pt]
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&= 2\cos^2\!v-1\\[5pt]
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&= 2b^2-1\,\textrm{.}
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\end{align}</math>}}

Current revision

With the formula for double angles and the Pythagorean identity cos2v+sin2v=1, we can express cos2v in terms of cosv,

cos2v=cos2v−sin2v=cos2v−(1−cos2v)=2cos2v−1=2b2−1.
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