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The depth of water(d metres) in a harbour is given by the formula d = a+bsin(ct) where a, b and c are constants, and t is the time in hours after midnight. It is known that both b and c are non-zero and 20<c<35. If t=midnight, d=5; if t=noon, d=5; if t=1300, d=7; and if t=1400, d=8.46. Using the pieces of information, find the values of a,b and c

I already found the a, which turned out to be 5 and it's right. I found it by using the formula 5=a+bsin(c0) and since anytime sin is times by 0, you get 0, then a=5.

asked by Jessica
  1. This is a 9th grade math problem by the way.

    posted by Jessica
  2. So, now you know that

    d(t) = 5+b sin(ct)
    Now plug in the values you know:

    b sin(12c) = 0
    b sin(1300c) = 2
    b sin(1400c) = 3.46

    we can see that 12c is a multiple of π.
    So,
    sin(1300c) = sin(1296c+4c) = sin(4c)
    sin(1400c) = sin(1392c+8c) = sin(8c)

    Hmmm. I'm stuck. 4c and 8c are both multiples of π/3, which have the same value for the sine. So, how can be be different?

    Am I off base here?


    posted by Steve

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