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In a certain sequence of numbers, each term after the first is found by doubling and then adding $3$ to the previous term. If the seventh term of the sequence is $125$, what is the first term?
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Apr 27, 2019
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In a certain sequence of numbers, each term after the first is found by doubling and then adding $3$ to the previous term. If the seventh term of the sequence is $125$, what is the first term?
Mathematics
high-school
PiotrK
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we know that
an=[a(n-1)]*2+3
if a7=125
a) Find a1
step 1
find a6
a7=a6*2+3--------> a6=(a7-3)/2-----> a6=(125-3)/2----> a6=61
step 2
find a5
a6=a5*2+3--------> a5=(a6-3)/2-----> a5=(61-3)/2----> a5=29
step 3
find a4
a5=a4*2+3--------> a4=(a5-3)/2-----> a4=(29-3)/2----> a4=13
step 4
find a3
a4=a3*2+3--------> a3=(a4-3)/2-----> a3=(13-3)/2----> a3=5
step 5
find a2
a3=a2*2+3--------> a2=(a3-3)/2-----> a2=(5-3)/2----> a2=1
step 6
find a1
a2=a1*2+3--------> a1=(a2-3)/2-----> a1=(1-3)/2----> a1=-1
the answer is
a1=-1
Arivero
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May 4, 2019
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Arivero
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