For Polynomial Lovers 2

Algebra Level 5

Suppose f ( x ) , g ( x ) f(x),g(x) and h ( x ) h(x) are polynomials such that

f ( x ) g ( x ) + h ( x ) = { 1 , if x < 1 3 x + 2 , if 1 x 0 2 x + 2 , if x > 0 \large |f(x)|-|g(x)|+|h(x)|=\begin{cases} -1,\quad \text{if } x<-1\\ 3x+2,\quad \text{if } -1 \leq x \leq 0\\ -2x+2,\quad \text{if } x>0 \end{cases}\\

Find the value of g ( 2 ) g(2) .

This is the part of Polynomialism .


The answer is 5.

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1 solution

Ravi Dwivedi
Jul 12, 2015

Suppose f ( x ) g ( x ) + h ( x ) = F ( x ) |f(x)|-|g(x)|+|h(x)|=F(x)

And since x = 1 x=-1 and x = 0 x=0 are critical points of F ( x ) F(x) we can write it as

F ( x ) = a x + 1 + b x + c x + d \large F(x)=a|x+1|+b|x|+cx+d , where a a , b b , c c , d d are constants.

F ( x ) = { ( c a b ) x + d a , x < 1 ( a + c b ) x + a + d , 1 x 0 ( a + b + c ) x + a + d , x 0 \large F(x) =\begin{cases} (c-a-b)x+d-a, \quad x<-1\\ (a+c-b)x+a+d,\quad -1 \leq x \leq 0\\ (a+b+c)x+a+d,\quad x\geq 0\end{cases}

On comparing with the given we get a = 3 2 , b = 5 2 , c = 1 , d = 1 2 a=\frac{3}{2},b=\frac{-5}{2},c=-1,d=\frac{1}{2}

This gives f ( x ) = 3 x + 3 2 , g ( x ) = 5 x 2 , h ( x ) = x + 1 2 f(x)= \frac{3x+3}{2}, g(x)= \frac{5x}{2}, h(x)= -x+ \frac{1}{2}

Note that by symmetry f ( x ) f(x) and h ( x ) h(x) may be interchanged

And finally g ( 2 ) = 5 g(2)= \boxed{5}

Moderator note:

As previously explained, you should mention that by the symmetry of f , h f, h , there are actually alternative solutions.

Due to symmetry in f and h last time there was a flaw in this question.I have deleted that created this new one

Sorry all of u for inconvenience and thanks for challenge master for pointing it out

Ravi Dwivedi - 5 years, 11 months ago

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