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If n is an integer, f(n) = f(n – 1) – n, and f(4) = 10.

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Guest
 Post subject: If n is an integer, f(n) = f(n – 1) – n, and f(4) = 10.
 Post Posted: Sun Jul 13, 2008 7:22 pm 
If n is an integer, f(n) = f(n – 1) – n, and f(4) = 10. What is the value of f(6)?

-1
0
1
2
4

Why in this problem do you not solve for f using F(4) equals 10.
What I did is this 10=F(4-1) - 4 and sovled for F.
but i got a 14/3=F which when used to tru and solve for f(6) does not work.
please help.
Thanks,
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Sudoku
 Post subject: Answer
 Post Posted: Sun Jul 13, 2008 9:41 pm 
I guess you are reading the question qrong. It is function of n not f*n


If n is an integer, f(n) = f(n – 1) – n, and f(4) = 10. What is the value of f(6)?

f(6)=f(5)-6 ...(A)
f(5)=f(4)-5
f(5)=10-5
f(5)=5

Back to A

f(6)=5-6 = -1
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RonPurewal
 Post subject: Re: If n is an integer, f(n) = f(n – 1) – n, and f(4) = 10.
 Post Posted: Sat Jul 19, 2008 4:03 am 
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ManhattanGMAT Staff


Posts: 7146
Anonymous wrote:
If n is an integer, f(n) = f(n – 1) – n, and f(4) = 10. What is the value of f(6)?

-1
0
1
2
4

Why in this problem do you not solve for f using F(4) equals 10.
What I did is this 10=F(4-1) - 4 and sovled for F.
but i got a 14/3=F which when used to tru and solve for f(6) does not work.
please help.
Thanks,


yeah, make sure you have a firm grip on the basics of "f notation" for functions. in particular, f(4) means that you plug 4 into a predetermined operation and see what comes out, NOT that you multiply 4 times a mysterious shape-changing number called "f".

this is the same way that other operators work, by the way. for instance, if i write √9 = 3, you clearly can't read this as "√" times 9 = 3, and therefore "√" must be equal to 1/3. instead, you understand that "√" is an operator - i.e., it takes the number you plug into it and transforms that number into something else.
the f in f(x) does the same thing as "√": it's an operator, which accepts a number plugged into it and spits out another number. if you read it as multiplication, you're setting yourself up for some big trouble.
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