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Given that 
sin A=(sqrt7)/(6) and 
tan B=sqrt3, and that angles 
A and 
B are both in Quadrant I, find the exact value of 
cos(A-B), in simplest radical form.
Answer:

Given that sinA=76 \sin A=\frac{\sqrt{7}}{6} and tanB=3 \tan B=\sqrt{3} , and that angles A A and B B are both in Quadrant I, find the exact value of cos(AB) \cos (A-B) , in simplest radical form.\newlineAnswer:

Full solution

Q. Given that sinA=76 \sin A=\frac{\sqrt{7}}{6} and tanB=3 \tan B=\sqrt{3} , and that angles A A and B B are both in Quadrant I, find the exact value of cos(AB) \cos (A-B) , in simplest radical form.\newlineAnswer:
  1. Find cosA\cos A: Use the Pythagorean identity to find cosA\cos A.\newlineSince sinA=76\sin A = \frac{\sqrt{7}}{6}, we can use the identity sin2A+cos2A=1\sin^2 A + \cos^2 A = 1 to find cosA\cos A.\newlinecos2A=1sin2A\cos^2 A = 1 - \sin^2 A\newlinecos2A=1(76)2\cos^2 A = 1 - \left(\frac{\sqrt{7}}{6}\right)^2\newlinecos2A=1736\cos^2 A = 1 - \frac{7}{36}\newlinecos2A=3636736\cos^2 A = \frac{36}{36} - \frac{7}{36}\newlinecos2A=2936\cos^2 A = \frac{29}{36}\newlinecosA\cos A00\newlinecosA\cos A11
  2. Find cosB\cos B: Use the identity 1+tan2B=sec2B1 + \tan^2 B = \sec^2 B to find cosB\cos B. Since tanB=3\tan B = \sqrt{3}, we can find secB\sec B and then use the reciprocal identity to find cosB\cos B. sec2B=1+tan2B\sec^2 B = 1 + \tan^2 B sec2B=1+(3)2\sec^2 B = 1 + (\sqrt{3})^2 sec2B=1+3\sec^2 B = 1 + 3 sec2B=4\sec^2 B = 4 1+tan2B=sec2B1 + \tan^2 B = \sec^2 B00 1+tan2B=sec2B1 + \tan^2 B = \sec^2 B11 1+tan2B=sec2B1 + \tan^2 B = \sec^2 B22 1+tan2B=sec2B1 + \tan^2 B = \sec^2 B33
  3. Find cos(AB)\cos(A-B): Use the cosine difference identity to find cos(AB)\cos(A-B). The identity is cos(AB)=cosAcosB+sinAsinB\cos(A-B) = \cos A \cdot \cos B + \sin A \cdot \sin B. cos(AB)=29612+7632\cos(A-B) = \frac{\sqrt{29}}{6} \cdot \frac{1}{2} + \frac{\sqrt{7}}{6} \cdot \frac{\sqrt{3}}{2} cos(AB)=2912+7312\cos(A-B) = \frac{\sqrt{29}}{12} + \frac{\sqrt{7} \cdot \sqrt{3}}{12} cos(AB)=2912+2112\cos(A-B) = \frac{\sqrt{29}}{12} + \frac{\sqrt{21}}{12} cos(AB)=29+2112\cos(A-B) = \frac{\sqrt{29} + \sqrt{21}}{12}

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