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math

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can someone please help me with this question.

Given the funtion f(x)= 3x-9
a. find the rate of change between the two stated values for x: 7 to 8

b. find the equation of a secant line containing the given points: (7,f(7)) and (8,f(8))

I think I have (a)answered, just need help with b; please.

  • math - ,

    f(7) = 21-9 = 12
    f(8) = 24-9 = 15

    so the change in the range = 15-12 = 3
    change in domain = 8-7 = 1

    for every change of 1 in the x's there will be a change of 3 in the y's
    or
    rate of change is 3/1 = 3

    b) from above, slope = 3
    y = 3x + b
    sub in (7,12)
    12 = 3(7) + b
    b = -9
    y = 3x - 9 ahhh, the same as the original equation !

    (since y = 3x-9 is a straight line, of course we are going to end up with the same equation for a "secant" between any two points on a straight line)

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