Set \(\displaystyle{f{{\left({x}\right)}}}={x}^{{{2}}}+{a}{x}+{b}\). Prove \(\displaystyle\max{\left\lbrace{\left|{f{{\left(-{1}\right)}}}\right|},\ {\left|{f{{\left({0}\right)}}}\right|},\ {\left|{f{{\left({1}\right)}}}\right|}\right\rbrace}\geq{\frac{{{1}}}{{{2}}}}\)

alparcero97oy

alparcero97oy

Answered question

2022-04-01

Set f(x)=x2+ax+b.
Prove max{|f(1)|, |f(0)|, |f(1)|}12

Answer & Explanation

Esteban Sloan

Esteban Sloan

Beginner2022-04-02Added 21 answers

Step 1
Let
max{|f(1)|,|f(0)|,|f(1)|}=k.
Thus, by your work
2=(2b)+(1a+b)+(1+a+b)
2|b|+|1a+b|+|1a+b|2k+k+k=4k,
which gives
k12
Carter Lin

Carter Lin

Beginner2022-04-03Added 13 answers

Step 1
Prove by contradiction. Suppose the maximum is less than 12.
Then |1a+b|<12 and |1+a+b|<12.
Can you use these to show that |2(1+b)|<1?
If you do that you get 1=|(1+b)b|<12+12=1 which is a contradiction.

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