It is 2018, is it not?

Algebra Level 3

There is a 3rd degree polynomial P ( x ) P(x) , such that

P ( 1 ) = 2018 P ( 2 ) = 2018 P ( 3 ) = 2018 P ( 4 ) = 1982 \begin{aligned} P(1) & =2018 \\ P(2) & =2018 \\ P(3) & =2018 \\ P(4) & =1982 \end{aligned}

What is P ( 0 ) P(0) ?

2018 2048 2054 2040

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2 solutions

Ethan Song
Jan 10, 2018

Define Q ( x ) Q(x) such that Q ( x ) = P ( x ) 2018 Q(x)=P(x)-2018 .

Thus,

  • Q ( 1 ) = 0 Q(1)=0

  • Q ( 2 ) = 0 Q(2)=0

  • Q ( 3 ) = 0 Q(3)=0

  • Q ( 4 ) = 36 Q(4)=-36

Because Q ( 1 ) = 0 Q(1)=0 , then ( x 1 ) (x-1) is a factor of the polynomial Q ( x ) Q(x) . The same logic holds for Q ( 2 ) Q(2) and Q ( 3 ) Q(3) .

Since Q ( x ) Q(x) is a three degree polynomial, (as we know from P ( x ) P(x) ), then Q ( x ) = h ( x 1 ) ( x 2 ) ( x 3 ) Q(x)=h(x-1)(x-2)(x-3) , where h is some constant.

Plugging x = 4 x=4 in, we get h = 6 Q ( x ) = 6 ( x 1 ) ( x 2 ) ( x 3 ) h=-6 \longrightarrow Q(x)=-6(x-1)(x-2)(x-3)

Therefore, P ( x ) = Q ( x ) + 2018 = 6 ( x 1 ) ( x 2 ) ( x 3 ) + 2018 P(x)=Q(x)+2018=-6(x-1)(x-2)(x-3)+2018 .

Plugging in x = 0 x=0 , we get an answer of 36 + 2018 = 2054 36+2018=\boxed{2054}

Chew-Seong Cheong
Jan 11, 2018

We note that P ( x ) = k ( x 1 ) ( x 2 ) ( x 3 ) + 2018 P(x) = k(x-1)(x-2)(x-3) + 2018 satisfies P ( 1 ) = P ( 2 ) = P ( 3 ) = 2018 P(1)=P(2)=P(3) = 2018 . If P ( 4 ) = 1982 P(4)=1982 , then:

k ( 4 1 ) ( 4 2 ) ( 4 3 ) + 2018 = 1982 6 k = 1982 2018 k = 6 P ( x ) = 2018 6 ( x 1 ) ( x 2 ) ( x 3 ) P ( 0 ) = 2018 6 ( 6 ) = 2054 \begin{aligned} k(4-1)(4-2)(4-3)+2018 & = 1982 \\ 6 k & = 1982 - 2018 \\ \implies k & = - 6 \\ \implies P(x) & = 2018 - 6(x-1)(x-2)(x-3) \\ P(0) & = 2018 -6(-6) = \boxed{2054} \end{aligned}

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