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Quadratic Equation and Inequalities question

2011 · Shift 1 · Q26
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  5. /2011 · Shift 1 · Q26

Quadratic Equation and Inequalities question

2011 · Shift 1 · Q26

JEE AdvancedMathematicsQuadratic Equation and InequalitiesNumerical+4 / −1
The minimum value of the sum of real numbers a−5, a−4, 3a−3, 1, a8{a^{ - 5}},\,{a^{ - 4}},\,3{a^{ - 3}},\,1,\,{a^8}a−5,a−4,3a−3,1,a8 and a10{a^{10}}a10 where a>0a \gt 0a>0 is
Numerical answer
View written solutionFree

Correct answer: 8

We need to minimize S=a−5+a−4+3a−3+1+a8+a10,  a>0.S=a^{-5}+a^{-4}+3a^{-3}+1+a^8+a^{10}, \,\, a>0.S=a−5+a−4+3a−3+1+a8+a10,a>0.

We will rewrite the expression in a form where AM-GM can be applied neatly.

1. Group the terms

Notice that a−5+a10a^{-5}+a^{10}a−5+a10 and a−4+a8a^{-4}+a^8a−4+a8 are natural pairs, because their exponents can be balanced.

Also, 3a−3=a−3+a−3+a−3.3a^{-3}=a^{-3}+a^{-3}+a^{-3}.3a−3=a−3+a−3+a−3.

So, S=(a−5+a10)+(a−4+a8)+(a−3+a−3+a−3)+1.S=(a^{-5}+a^{10})+(a^{-4}+a^8)+(a^{-3}+a^{-3}+a^{-3})+1.S=(a−5+a10)+(a−4+a8)+(a−3+a−3+a−3)+1.

2. Apply AM-GM to each pair

For positive numbers, AM-GM gives x+y≥2xy.x+y\ge 2\sqrt{xy}.x+y≥2xy​.

Pair 1:

a−5+a10≥2a−5⋅a10=2a5=2a5/2.a^{-5}+a^{10}\ge 2\sqrt{a^{-5}\cdot a^{10}}=2\sqrt{a^5}=2a^{5/2}.a−5+a10≥2a−5⋅a10​=2a5​=2a5/2.

Pair 2:

a−4+a8≥2a−4⋅a8=2a4=2a2.a^{-4}+a^8\ge 2\sqrt{a^{-4}\cdot a^8}=2\sqrt{a^4}=2a^2.a−4+a8≥2a−4⋅a8​=2a4​=2a2.

Thus, S≥2a5/2+2a2+3a−3+1.S\ge 2a^{5/2}+2a^2+3a^{-3}+1.S≥2a5/2+2a2+3a−3+1.

This is true, but not immediately enough to get a clean minimum. So let us instead look for a point where all terms become simple and check whether AM-GM can directly produce a constant lower bound.

3. Test the natural equality point a=1a=1a=1

At a=1a=1a=1, S=1+1+3+1+1+1=8.S=1+1+3+1+1+1=8.S=1+1+3+1+1+1=8.

So the minimum is at most 888.

Now we prove that S≥8S\ge 8S≥8 for all a>0a>0a>0.

4. Prove the lower bound S≥8S\ge 8S≥8

Using AM-GM on suitable grouped terms:

First group:

a−5+a−4+a−3≥3a−5a−4a−33=3a−4.a^{-5}+a^{-4}+a^{-3}\ge 3\sqrt[3]{a^{-5}a^{-4}a^{-3}}=3a^{-4}.a−5+a−4+a−3≥33a−5a−4a−3​=3a−4.

This alone is not enough. A better grouping is to compare each term with 1 through AM-GM:

For positive xxx, x+1≥2x.x+1\ge 2\sqrt{x}.x+1≥2x​.

Apply to a−5a^{-5}a−5 and a10a^{10}a10: a−5+a10≥2a5=2a5/2.a^{-5}+a^{10}\ge 2\sqrt{a^5}=2a^{5/2}.a−5+a10≥2a5​=2a5/2.

Apply to a−4a^{-4}a−4 and a8a^8a8: a−4+a8≥2a4=2a2.a^{-4}+a^8\ge 2\sqrt{a^4}=2a^2.a−4+a8≥2a4​=2a2.

So S≥2a5/2+2a2+3a−3+1.S\ge 2a^{5/2}+2a^2+3a^{-3}+1.S≥2a5/2+2a2+3a−3+1.

Now at a=1a=1a=1, this becomes 888, and the equality conditions of both AM-GM inequalities require a−5=a10⇒a15=1⇒a=1,a^{-5}=a^{10} \Rightarrow a^{15}=1 \Rightarrow a=1,a−5=a10⇒a15=1⇒a=1, a−4=a8⇒a12=1⇒a=1.a^{-4}=a^8 \Rightarrow a^{12}=1 \Rightarrow a=1.a−4=a8⇒a12=1⇒a=1.

Hence a=1a=1a=1 is the common equality point.

To ensure it is indeed the minimum, define f(a)=a−5+a−4+3a−3+1+a8+a10.f(a)=a^{-5}+a^{-4}+3a^{-3}+1+a^8+a^{10}.f(a)=a−5+a−4+3a−3+1+a8+a10.

Differentiate: f′(a)=−5a−6−4a−5−9a−4+8a7+10a9.f'(a)=-5a^{-6}-4a^{-5}-9a^{-4}+8a^7+10a^9.f′(a)=−5a−6−4a−5−9a−4+8a7+10a9.

At a=1a=1a=1, f′(1)=−5−4−9+8+10=0.f'(1)=-5-4-9+8+10=0.f′(1)=−5−4−9+8+10=0.

Second derivative: f′′(a)=30a−7+20a−6+36a−5+56a6+90a8>0(a>0).f''(a)=30a^{-7}+20a^{-6}+36a^{-5}+56a^6+90a^8>0 \quad (a>0).f′′(a)=30a−7+20a−6+36a−5+56a6+90a8>0(a>0).

So fff is strictly convex on (0,∞)(0,\infty)(0,∞), and the critical point a=1a=1a=1 is the unique global minimum.

Therefore, 8\boxed{8}8​

5. Compare with stored answer

Stored correct answer: 888

Our derived answer is also 888, so they agree.

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