Posted by **Jake** on Sunday, February 19, 2012 at 5:55pm.

if f is an exponential function, and f(5)=20 and f(9) = 14, use the add-multiply property to calculate f(13),f(17), and f(21). Show your method

- Math -
**bobpursley**, Sunday, February 19, 2012 at 6:08pm
there is a difference of x=+4 between 5, 9

There is a multiply difference of 14/20=.7 between the two.

f(13)=14*.7

f(17)=14*.7*.7

f(21)=14(.7*.7*.7)

- Math -
**Reiny**, Sunday, February 19, 2012 at 6:09pm
let f(x) = a(b^x) or

y = a(b^x)

given: when x=5, y=20

20 = a b^5

given: when x=9, y=14

14 = a b^9

divide the 2nd by the first:

b^4 = 14/20 = 0.7

b = (0.7)^.25 = appr .91469

sub into 20 = a b^5

20 = a(.91469)^5

a = 31.236

f(x) = 31.236 (.91469)^x

f(13) = 31.236(.91469)^13 = 9.8

you do the button-pushing for the rest

Note, I used the memory locations of my calculator to store all the supplied decimal values for both a and b

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