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Given that 
tan x=sqrt3 and 
cos y=(sqrt2)/(2), and that angles 
x and 
y are both in Quadrant I, find the exact value of 
cos(x+y), in simplest radical form.
Answer:

Given that tanx=3 \tan x=\sqrt{3} and cosy=22 \cos y=\frac{\sqrt{2}}{2} , and that angles x x and y y are both in Quadrant I, find the exact value of cos(x+y) \cos (x+y) , in simplest radical form.\newlineAnswer:

Full solution

Q. Given that tanx=3 \tan x=\sqrt{3} and cosy=22 \cos y=\frac{\sqrt{2}}{2} , and that angles x x and y y are both in Quadrant I, find the exact value of cos(x+y) \cos (x+y) , in simplest radical form.\newlineAnswer:
  1. Find sinx\sin x: Use the given information to find the values of sinx\sin x and siny\sin y. Since tanx=3\tan x = \sqrt{3}, we know that sinxcosx=3\frac{\sin x}{\cos x} = \sqrt{3}. We also know that cos2x+sin2x=1\cos^2 x + \sin^2 x = 1 (Pythagorean identity). We need to find sinx\sin x.
  2. Calculate cosx\cos x: Calculate cosx\cos x using the Pythagorean identity.\newlineSince tanx=sinxcosx=3\tan x = \frac{\sin x}{\cos x} = \sqrt{3}, we can assume a right triangle where the opposite side is 3\sqrt{3} and the adjacent side is 11 (since tan is opposite over adjacent). Therefore, the hypotenuse is 12+(3)2=1+3=4=2\sqrt{1^2 + (\sqrt{3})^2} = \sqrt{1 + 3} = \sqrt{4} = 2. So, cosx=adjacenthypotenuse=12\cos x = \frac{\text{adjacent}}{\text{hypotenuse}} = \frac{1}{2}.
  3. Calculate sinx\sin x: Calculate sinx\sin x using the Pythagorean identity.\newlineNow that we have cosx\cos x, we can use the identity sin2x+cos2x=1\sin^2 x + \cos^2 x = 1 to find sinx\sin x. We have cosx=12\cos x = \frac{1}{2}, so cos2x=(12)2=14\cos^2 x = (\frac{1}{2})^2 = \frac{1}{4}. Therefore, sin2x=114=34\sin^2 x = 1 - \frac{1}{4} = \frac{3}{4}, and sinx=34=32\sin x = \sqrt{\frac{3}{4}} = \frac{\sqrt{3}}{2}.
  4. Find siny\sin y: Use the given information to find siny\sin y. We are given cosy=22\cos y = \frac{\sqrt{2}}{2}. Using the Pythagorean identity sin2y+cos2y=1\sin^2 y + \cos^2 y = 1, we can find siny\sin y. We have cos2y=(22)2=24=12\cos^2 y = \left(\frac{\sqrt{2}}{2}\right)^2 = \frac{2}{4} = \frac{1}{2}. Therefore, sin2y=112=12\sin^2 y = 1 - \frac{1}{2} = \frac{1}{2}, and siny=12=22\sin y = \sqrt{\frac{1}{2}} = \frac{\sqrt{2}}{2}.
  5. Use angle sum identity: Use the angle sum identity for cosine to find cos(x+y)\cos(x+y). The angle sum identity for cosine is cos(x+y)=cosxcosysinxsiny\cos(x+y) = \cos x \cdot \cos y - \sin x \cdot \sin y. We have already found cosx\cos x, cosy\cos y, sinx\sin x, and siny\sin y.
  6. Substitute and simplify: Substitute the values into the angle sum identity and simplify.\newlinecos(x+y)=12223222\cos(x+y) = \frac{1}{2} \cdot \frac{\sqrt{2}}{2} - \frac{\sqrt{3}}{2} \cdot \frac{\sqrt{2}}{2}\newlinecos(x+y)=2464\cos(x+y) = \frac{\sqrt{2}}{4} - \frac{\sqrt{6}}{4}\newlinecos(x+y)=264\cos(x+y) = \frac{\sqrt{2} - \sqrt{6}}{4}

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