Difference between revisions of "1994 AIME Problems/Problem 3"

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== Solution ==
 
== Solution ==
{{solution}}
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<math>f(94)=94^2-f(93)=94^2-93^2+f(92)=94^2-93^2+92^2-f(91)=\ldots</math>
  
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<math>f(94) = (94^2-93^2) + (92^2-91^2) +\ldots+ (22^2-21^2)+ 20^2-f(19) = 94+93+\ldots+21+400-94 </math>
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<math>f(94) = 4561</math>
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So, the remainder is 561.
 
== See also ==
 
== See also ==
 
{{AIME box|year=1994|num-b=2|num-a=4}}
 
{{AIME box|year=1994|num-b=2|num-a=4}}

Revision as of 05:24, 31 December 2007

Problem

The function $f_{}^{}$ has the property that, for each real number $x,\,$

$f(x)+f(x-1) = x^2\,$

.

If $f(19)=94,\,$ what is the remainder when $f(94)\,$ is divided by 1000?

Solution

$f(94)=94^2-f(93)=94^2-93^2+f(92)=94^2-93^2+92^2-f(91)=\ldots$

$f(94) = (94^2-93^2) + (92^2-91^2) +\ldots+ (22^2-21^2)+ 20^2-f(19) = 94+93+\ldots+21+400-94$

$f(94) = 4561$


So, the remainder is 561.

See also

1994 AIME (ProblemsAnswer KeyResources)
Preceded by
Problem 2
Followed by
Problem 4
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
All AIME Problems and Solutions