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

Entered answer:

Solution

✅ Correct Answer: 8

We need to find the smallest integer nn that satisfies the inequality n3−11n2+32n−28>0n^3 - 11n^2 + 32n - 28 > 0.


The key insight is to factor the polynomial n3−11n2+32n−28n^3 - 11n^2 + 32n - 28.

Let's check if n=2n = 2 is a root:

23−11(22)+32(2)−28=8−44+64−28=02^3 - 11(2^2) + 32(2) - 28 = 8 - 44 + 64 - 28 = 0

Since n=2n = 2 makes the polynomial equal to zero, (n−2)(n-2) is a factor.

Using polynomial division, we find that:

n3−11n2+32n−28=(n−2)2(n−7)n^3 - 11n^2 + 32n - 28 = (n-2)^2(n-7)

Notice that n=2n = 2 is actually a repeated root (appears twice), which is why we get (n−2)2(n-2)^2 in the factorization.


Our inequality becomes:

(n−2)2(n−7)>0(n-2)^2(n-7) > 0


To solve this inequality, we need to understand when the product (n−2)2(n−7)(n-2)^2(n-7) is positive.

Key insight: (n−2)2(n-2)^2 is always non-negative for any real number nn, and equals zero only when n=2n = 2.

Let's analyze the sign of (n−7)(n-7):

When n<7n < 7: (n−7)<0(n-7) < 0 (negative)

When n=7n = 7: (n−7)=0(n-7) = 0 (zero)

When n>7n > 7: (n−7)>0(n-7) > 0 (positive)


For the product (n−2)2(n−7)>0(n-2)^2(n-7) > 0 to be positive:

When n<2n < 2: (n−2)2(n-2)^2 is positive and (n−7)(n-7) is negative, so the product is negative

When n=2n = 2: (n−2)2(n-2)^2 is zero and (n−7)(n-7) is negative, so the product is zero

When 2<n<72 < n < 7: (n−2)2(n-2)^2 is positive and (n−7)(n-7) is negative, so the product is negative

When n=7n = 7: (n−2)2(n-2)^2 is positive and (n−7)(n-7) is zero, so the product is zero

When n>7n > 7: (n−2)2(n-2)^2 is positive and (n−7)(n-7) is positive, so the product is positive


The inequality (n−2)2(n−7)>0(n-2)^2(n-7) > 0 is satisfied when n>7n > 7.

The smallest integer greater than 7 is 8.

Therefore, the smallest integer nn is 8.

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