Q. Given that tanx=3 and cosy=22, and that angles x and y are both in Quadrant I, find the exact value of cos(x+y), in simplest radical form.Answer:
Find sinx: Use the given information to find the values of sinx and siny. Since tanx=3, we know that cosxsinx=3. We also know that cos2x+sin2x=1 (Pythagorean identity). We need to find sinx.
Calculate cosx: Calculate cosx using the Pythagorean identity.Since tanx=cosxsinx=3, we can assume a right triangle where the opposite side is 3 and the adjacent side is 1 (since tan is opposite over adjacent). Therefore, the hypotenuse is 12+(3)2=1+3=4=2. So, cosx=hypotenuseadjacent=21.
Calculate sinx: Calculate sinx using the Pythagorean identity.Now that we have cosx, we can use the identity sin2x+cos2x=1 to find sinx. We have cosx=21, so cos2x=(21)2=41. Therefore, sin2x=1−41=43, and sinx=43=23.
Find siny: Use the given information to find siny. We are given cosy=22. Using the Pythagorean identity sin2y+cos2y=1, we can find siny. We have cos2y=(22)2=42=21. Therefore, sin2y=1−21=21, and siny=21=22.
Use angle sum identity: Use the angle sum identity for cosine to find cos(x+y). The angle sum identity for cosine is cos(x+y)=cosx⋅cosy−sinx⋅siny. We have already found cosx, cosy, sinx, and siny.
Substitute and simplify: Substitute the values into the angle sum identity and simplify.cos(x+y)=21⋅22−23⋅22cos(x+y)=42−46cos(x+y)=42−6
More problems from Find trigonometric ratios using a Pythagorean or reciprocal identity