Which is an exponential decay function? f(x)=3/4(7/4)^x
fx) = 2/3(4/5)^-x
f(x) = 3/2(8/7)^-x
f(x)= 1/3(-9/2)^x

c)3/2(8/7)^x

Step-by-step explanation:

Evaluate the functions. 

1. Because 7/4 > 1 and the exponent is positive,

  the function does not decay.

2.Because 4/5 < 1 and the exponent is negative,

   the function does not decay.

3. Because 8/7 > 1 and the exponent is negative,

   the function decays.

4. Because 9/2 > 1 and the exponent is positive,

   the function does not decay.

A composite plot the functions verifies the answer.

If all the x's are exponents, then 3/2 (8/7)^-x is the only one that decays as x gets bigger.  That's because the base is greater than 1 and the x is negative.

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the answer is 40.

proof: