Consider the expression
(a^2 + a + 1) (b^2 + b + 1) (c^2 + c + 1) (d^2 + d + 1) (e^2 + e + 1)
abcde
where a, b, c, d and e are positive numbers. The minimum value of the expression is
A. 3
B. 1
C. 10
D.100
E. 243
Answer: E
Minimun Value Expressions
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To minimize the value we need to minimize the numerator and maximize the denominator.coolhabhi wrote:Consider the expression
(a^2 + a + 1) (b^2 + b + 1) (c^2 + c + 1) (d^2 + d + 1) (e^2 + e + 1)
abcde
where a, b, c, d and e are positive numbers. The minimum value of the expression is
But if we try to maximize any factor of the denominator (say a), we will end up in increasing the numerator (a² + a + 1) more.
Hence, let us consider in minimizing the numerator only.
Each of the factors of the numerator will be minimized when a, b, c, d, and e will have the minimum possible value which is 1.
Hence, minimum possible value of the expression = (1 + 1 + 1)*(1 + 1 + 1)*(1 + 1 + 1)*(1 + 1 + 1)*(1 + 1 + 1)/1 = 3^5 = 243
The correct answer is E.
Anju Agarwal
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Proper Algebraic Method :
To minimize the expression, we need to minimize each of the factors of the expression like (a² + a + 1)/a, (b² + b + 1)/b, ... etc
It is obvious that if we can minimize any of the five factors, all the others can be minimized in the same way. Let us take the first one.
Now, (a² + a + 1)/a = (a + 1 + 1/a)
Hence, we need to minimize (a + 1/a).
Now, (a + 1/a)² = (a - 1/a)² + 4*a*(1/a) = (a - 1/a)² + 4
As, minimum possible value of (a - 1/a)² is zero, minimum possible value of (a + 1/a)² is 4. Hence, minimum possible value of (a + 1/a) is 2. [As a is a positive integer, we are discarding -2]
Hence, minimum possible value of (a² + a + 1)/a is (2 + 1) = 3.
Hence, minimum possible value of the given expression is 3^5 = 243
The correct answer is E.
To minimize the expression, we need to minimize each of the factors of the expression like (a² + a + 1)/a, (b² + b + 1)/b, ... etc
It is obvious that if we can minimize any of the five factors, all the others can be minimized in the same way. Let us take the first one.
Now, (a² + a + 1)/a = (a + 1 + 1/a)
Hence, we need to minimize (a + 1/a).
Now, (a + 1/a)² = (a - 1/a)² + 4*a*(1/a) = (a - 1/a)² + 4
As, minimum possible value of (a - 1/a)² is zero, minimum possible value of (a + 1/a)² is 4. Hence, minimum possible value of (a + 1/a) is 2. [As a is a positive integer, we are discarding -2]
Hence, minimum possible value of (a² + a + 1)/a is (2 + 1) = 3.
Hence, minimum possible value of the given expression is 3^5 = 243
The correct answer is E.
Anju Agarwal
Quant Expert, Gurome
Backup Methods : General guide on plugging, estimation etc.
Wavy Curve Method : Solving complex inequalities in a matter of seconds.
§ GMAT with Gurome § Admissions with Gurome § Career Advising with Gurome §
Quant Expert, Gurome
Backup Methods : General guide on plugging, estimation etc.
Wavy Curve Method : Solving complex inequalities in a matter of seconds.
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(a^2 + a + 1)/a= a+1/a + 1 Apply AM>=GM we get a+1/a > 2
Similarly for other expressions we get 3^5 = 243.
Similarly for other expressions we get 3^5 = 243.