Smallest integer making polynomial positive

CAT 2018 Slot 2 · Quantitative Ability · Medium · Polynomials

The smallest integer nn such that n3−11n2+32n−28>0n^3 - 11n^2 + 32n - 28 > 0 is

Answer

8

Explanation

Let P(n)=n3−11n2+32n−28P(n) = n^3 - 11n^2 + 32n - 28. Testing small integer roots: P(2)=8−44+64−28=0P(2) = 8 - 44 + 64 - 28 = 0. So (n−2)(n-2) is a factor.

Factoring P(n)P(n): P(n)=(n−2)(n2−9n+14)=(n−2)(n−2)(n−7)=(n−2)2(n−7)P(n) = (n-2)(n^2 - 9n + 14) = (n-2)(n-2)(n-7) = (n-2)^2(n-7)

We need (n−2)2(n−7)>0(n-2)^2(n-7) > 0. Since (n−2)2>0(n-2)^2 > 0 for all integers n≠2n \neq 2, the inequality holds when: n−7>0  ⟹  n>7n - 7 > 0 \implies n > 7

The smallest integer n>7n > 7 is 8.

Related Polynomials questions

Practise this under exam conditions

Sign in to solve it with a live timer, the on-screen CAT calculator, and streak and accuracy tracking across every question you attempt.

Solve in the workspace