A₆ =12
a₇ = ?? = x
a₈ 27
In this case x is the geometric mean & as per the formula
geometric mean of 2 numbers =√(a₆ * a₈) = √(12*27)
& x=18.
Proof:
Find the common ratio between a₇ & a₆ ==> = ( x) / 12
now find the common ration between a₈ & a₇ ===> =27 / (x)
x/12 = 27/x ===> x² = 12*27 ==> x² = 324 & x =√324 = 18
This is exponential decay which can be expressed as:
y=ab^t, y=final value, a=initial value, b=rate, t=time
In this case a=25000, r=(100-20)/100=0.8 so
y=25000(0.8^t), so in 4.5 years...
y=25000(0.8^4.5)
y≈$9158.93 (to nearest cent)
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