Let f ( x ) be the unique polynomial that satisfies
f ( n ) = i = 1 ∑ n i 2 3 , for all positive integers n .
What is the degree of f ( x ) ?
This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.
Good approach using the Method of differences , to rigorously determine the degree of the polynomial.
I didn't get it. can you help me out?
Well, Pranav A, your method is also good, as I also couldn't think of any other method than method of differences. Also, the solution of method of differences uses inductive reasoning to conclude that the following pattern also holds for degree 23. But your method uses deductive reasoning. Hats off!
Great solution! It almost took me a week to solve this problem and what I ended up with is a very long method. This is very compact and nicely done. Thanks for sharing! :)
use method of dumb luck
wonderful.
It is given that f(x) is a polynomial.
Let f ( x ) = i = 0 ∑ p a i x i where a p = 0 .
From the question, we have, f ( x ) − f ( x − 1 ) = x 2 3
Substituting f ( x ) and f ( x − 1 ) , we notice that the highest power of x remaining on LHS is p − 1 . Notice that a p x p cancels out.
Comparing both the sides, we find that p − 1 = 2 3 ⇒ p = 2 4 .
Therefore, the degree of f ( x ) is 24.
Recollecting sum of N natural numbers = N ( N + 1 ) / 2..............................
.Degree 1 higher than that of what we are summing
Sum of the series ( N ^ 2 ) where N starts from 1 is N ( N +1 ) ( 2 N + 1 ) / 6........................
Degree 1 higher than that of what we are summing
Sum of the series ( N ^ 3 ) where N starts from 1 is N ( N +1 ) N ( N + 1 ) / 4........................
Degree 1 higher than that of what we are summing
Thus here too the degree should be one higher than 23 which is 24
Problem Loading...
Note Loading...
Set Loading...
we all know that a sequence with a constant common difference, the general term is a one degree polynomial and the sum is 2 n ( n + 1 ) and it is a 2 degree polynomial. and the general term of a sequence with an increasing common difference is a two degree polynomial. the sum is 6 n ( n + 1 ) ( 2 n + 1 ) and it is a 3 degree polynomial. so we can conclude that the degree of the polynomial sum of a 23 degree polynomial general term is just by adding 1 degree to it. so it is 24.