Question

Angles A and B are located in the first quadrant such that sinA= 7/25 and cosB = 5/13. Determine the exact value for cos (A-B).

Answer 1

Answer:

  204/325

Step-by-step explanation:

You can work this a couple of ways. We expect you are probably expected to use trig identities.

  cos(A) = √(1 -sin²(A)) = 24/25

  sin(B) = √(1 -cos²(B)) = 12/13

cos(A -B) = cos(A)cos(B) +sin(A)sin(B) = (24/25)(5/13) +(7/25)(12/13)

  = (24·5 +7·12)/325

cos(A -B) = 204/325

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The other way to work this is using inverse trig functions. It is necessary to carry the full calculator precision if you want an exact answer.

  cos(A -B) = cos(arcsin(7/25) -arccos(5/13)) = cos(16.2602° -67.3801°)

  = cos(-51.1199°) ≈ 0.62769230 . . . . (last 6 digits repeating)

The denominators of 25 and 13 suggest that the desired fraction will have a denominator of 25·13 = 325, so we can multiply this value by 325 to see what we get.

  325·cos(A-B) = 204

so, the exact value is ...

  cos(A -B) = 204/325