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Sagot :
Expand cos(A - B) with the identity
cos(A - B) = cos(A) cos(B) + sin(A) sin(B)
A is in quadrant II, so sin(A) > 0, and B is in quadrant I, so sin(B) > 0. Using the Pythagorean identity, we get
cos²(A) + sin²(A) = 1 ⇒ sin(A) = + √(1 - (-3/5)²) = 4/5
cos²(B) + sin²(B) = 1 ⇒ sin(A) = + √(1 - (8/17)²) = 15/17
Then
cos(A - B) = (-3/5) × 8/17 + 4/5 × 15/17 = 36/85
cos (A - B) is 36/85
How to simply the identity
Expand cos(A - B) with the identity
You get, cos(A - B) = cos(A) cos(B) + sin(A) sin(B)
Since A is in quadrant II, so sin(A) > 0,
B is in quadrant I, so sin(B) > 0.
Using the Pythagorean identity, we get
cos²(A) + sin²(A) = 1
Make sin A the subject of formula
[tex]sin(A)^{2}[/tex] = ([tex]\sqrt{(1 - (-3/5}[/tex])²)
Find the square root of both sides, square root cancels square
[tex]sin A[/tex] = 4/5
Repeat the same for the second value
[tex]sin A^{2} = \sqrt{(1- 8/17)^2}[/tex]
[tex]sin A[/tex] = 15/17
Substitute values into cos(A - B)
cos(A - B) = cos(A) cos(B) + sin(A) sin(B) = (-3/5) * 8/17 + 4/5 * 15/17
cos (A - B) = 36/85
Therefore, cos (A - B) is 36/85
Learn more about trigonometric identities here:
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