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if cosA = 4/5; sin <0; cosB = 12/13; 0<B<90, determine sin(A-B)

thanks ... :)

  • Maths -

    The first pairing says that sinA is -4/5

    If cosB is 12/13, then sinB is 7/13

    Sin(A-B)=CosASinB-SinACosB, right?

  • Maths -

    yeah that's right, can u go on?

  • Maths -

    I could, but I wont. However, I will be happy to critique your work or thinking

  • Maths -

    Error: sinA=-3/5

    in all cases, sin^2 Theta + cos^2 Theta=1

  • Maths -

    well, i got:

    (-4/5)(12/13) - (4/5)(7/13)
    therefore

    (-48/65) - (28/65)

    therefore:

    answer = (-76/65)

  • Maths -

    from if cosA = 4/5; sin <0 we know that A is in the fourth quadrant and sinA = -3/5

    from cosB = 12/13; 0<B<90 we know sinB = 5/13

    sin(A-B) = sinAcosB - cosAsinB
    = (-3/5)(12/13) - (4/5)(5/13)
    = -56/65

  • Maths -

    change the Sin A in the first term, and it works.

  • Maths -

    i knew that was wrong, coz my answer should be in the 50's haha..thanks though, great help

    can i ask how you got sinA and sinB with that information :)

    no point in me just copying it i would like to know how you got it

  • Maths -

    In any angle, the sum of the squares of the cosines and sines is equal to one. Use that to solve.

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