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Solution 4.3:3c

From Förberedande kurs i matematik 1

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With the help of the Pythagorean identity, we can express cosv in terms of sinv,

cos2v+sin2v=1cosv=1sin2v. 

In addition, we know that the angle v lies between 2 and 2, i.e. either in the first or fourth quadrant, where angles always have a positive x-coordinate (cosine value); thus, we can conclude that

cosv=1sin2v=1a2.