Homework Help: math

Posted by ladybug on Saturday, September 15, 2012 at 11:28am.

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 - Reiny, Saturday, September 15, 2012 at 12:13pm
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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