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here are two geometic sequences: A: 1,6,36,36,216,1296... B: 50,150,450,1350,4050... after how many terms does A overtake B

 May 15, 2016

Best Answer 

 #2
avatar+37170 
+5

A sequence is 6^(n-1)     B sequence is 50(3^(n-1))

Set them equal and solve for n

 

6^(n-1) = 50(3 ^(n-1))

 Take log of both sides

 

(n-1)Log 6 = Log50 +(n-1)log3 

(n-1)(log6-log3) = log 50

n-1 = log50 /(log6-log3)

n = log50/(log6-log3) + 1

n= 6.64    This is where they would EQUAL,,,,the nex integer is where A>B           = 7

the integer after which A>B is 6

 May 15, 2016
edited by ElectricPavlov  May 15, 2016
 #2
avatar+37170 
+5
Best Answer

A sequence is 6^(n-1)     B sequence is 50(3^(n-1))

Set them equal and solve for n

 

6^(n-1) = 50(3 ^(n-1))

 Take log of both sides

 

(n-1)Log 6 = Log50 +(n-1)log3 

(n-1)(log6-log3) = log 50

n-1 = log50 /(log6-log3)

n = log50/(log6-log3) + 1

n= 6.64    This is where they would EQUAL,,,,the nex integer is where A>B           = 7

the integer after which A>B is 6

ElectricPavlov May 15, 2016
edited by ElectricPavlov  May 15, 2016
 #3
avatar+33661 
+5

Sequence A:  an = 6(n-1)  or  an = 2(n-1)*3(n-1)

 

Sequence B:  bn = 50*3(n-1)

 

For A to be larger than B:   2(n-1)*3(n-1)  >  50*3(n-1)   

 

or   2(n-1)  >  50  

 

The smallest value of n for which 2(n-1)  >  50  is n = 7  (as 25 = 32 and 26 = 64) 

 

Hence after 7 terms A is larger than B.

 May 15, 2016

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