My first problem

Algebra Level 4

F ( x ) F(x) is a monic cubic polynomial where

F ( 0 ) = 1 , F ( 17 ) = 5492 , F ( 23 ) = 13226. F(0)=1, F(17)=5492, F(23)=13226.

Find the sum of its coefficients.


The answer is 4.

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2 solutions

L e t t h e p o l y n o m i a l b e F ( x ) = x 3 + a x 2 + b x + c F ( 0 ) = 1 c = 1...... ( 1 ) F ( 17 ) = 5492 1 7 3 + 1 7 2 a + 17 b = 5491 = 17 323 1 7 2 + 17 a + b = 323 17 a + b = 34...... ( 2 ) F ( 23 ) = 13226 2 3 3 + 2 3 2 a + 23 b = 13225 = 23 575 2 3 2 + 23 a + b = 575 23 a + b = 46...... ( 3 ) S o l v i n g ( 2 ) a n d ( 3 ) w e g e t a = 2 a n d b = 0..... ( 4 ) L e a d i n g c o e f f i c i e n t i s 1 a n d f r o m ( 1 ) a n d ( 4 ) A n s w e r = 1 + a + b + c = 1 + 2 + 0 + 1 = 4 Let~ the ~polynomial ~be ~F(x) = x^3+ax^2 + bx + c \\F(0)=1~ \implies ~c=1......(1)\\ F(17)=5492 ~~ \implies ~ 17^3+17^2 a+ 17b =5491 =17*323\\\therefore ~17^2+17a+b=323 ~~ \implies ~17a + b = 34......(2)\\F(23)=13226 ~ \implies ~ 23^3+23^2a + 23b=13225=23*575\\\therefore ~23^2+23a+b=575 ~~ \implies ~23a + b = 46......(3)\\Solving~ (2) ~and~ (3) ~we ~get ~a=2~ and~ b=0 .....(4)\\ Leading~ coefficient~ is ~1~ and ~from~ (1) ~ and ~ (4)~ \\Answer = 1+a+b+c=1+2+0+1= \boxed{ \color{#20A900} {\huge {4} }}

good solution for an easy problem

Manit Kapoor - 6 years, 5 months ago
Chew-Seong Cheong
Jan 15, 2015

Let F ( x ) = x 3 + a x 2 + b x + c F(x) = x^3+ax^2+bx+c . Then, we have:

{ F ( 0 ) = 0 + 0 + 0 + c = 1 c = 1 F ( 17 ) = 4913 + 289 a + 17 b + 1 = 5492 289 a + 17 b = 578 . . . ( 1 ) F ( 23 ) = 12167 + 529 a + 23 b + 1 = 13226 529 a + 23 b = 1058 . . . ( 2 ) \begin{cases} F(0) = 0+0+0+c = 1 & \Rightarrow c = 1 & \\ F(17) = 4913+289a+17b +1=5492 & \Rightarrow 289a+17b = 578 &...(1)\\ F(23) = 12167+529a+23b+1=13226 & \Rightarrow 529a+23b = 1058 &...(2) \end{cases}

{ E q . 1 × 23 : 6647 a + 391 b = 13294 . . . ( 1 a ) E q . 2 × 17 : 8993 a + 391 b = 17986 . . . ( 2 a ) \begin{cases} Eq.1 \times 23: & 6647a+391b=13294 &...(1a) \\ Eq.2 \times 17: & 8993a+391b=17986 &...(2a) \end{cases}

E q . 2 a E q . 1 a : 2346 a + 0 = 4692 a = 2 Eq.2a-Eq.1a: 2346a +0 = 4692\quad \Rightarrow a = 2

Substituting a = 2 \space a=2\space in E q . 1 : 578 + 17 b = 578 b = 0 \space Eq.1: 578 +17b = 578 \quad \Rightarrow b = 0

Therefore the sum of coefficients = 1 + a + b + c = 1 + 2 + 0 + 1 = 4 = 1+a+b+c = 1+2+0+1 = \boxed{4}

nice solution

Manit Kapoor - 6 years, 4 months ago

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