Smallest integer making polynomial positive

CAT 2018 Slot 2 · QA · Medium · Polynomials

The smallest integer nn such that n311n2+32n28>0n^3 - 11n^2 + 32n - 28 > 0 is

Answer

8

Explanation

Let P(n)=n311n2+32n28P(n) = n^3 - 11n^2 + 32n - 28. Testing small integer roots: P(2)=844+6428=0P(2) = 8 - 44 + 64 - 28 = 0. So (n2)(n-2) is a factor.

Factoring P(n)P(n): P(n)=(n2)(n29n+14)=(n2)(n2)(n7)=(n2)2(n7)P(n) = (n-2)(n^2 - 9n + 14) = (n-2)(n-2)(n-7) = (n-2)^2(n-7)

We need (n2)2(n7)>0(n-2)^2(n-7) > 0. Since (n2)2>0(n-2)^2 > 0 for all integers n2n \neq 2, the inequality holds when: n7>0    n>7n - 7 > 0 \implies n > 7

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

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