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Question

The minimum value of (6+x)(11+x)(2+x), x0 is

A
45
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B
25
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C
5
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D
15
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Solution

The correct option is B 25
Let y=(6+x)(11+x)2+x=x2+17x+662+x
y=(2+x)(2x+17)(x2+17x+66)(2+x)2
y=2x2+21x+34x217x66(2+x)2
y=x2+4x32(2+x)2
y=(x+8)(x4)(2+x)2
Critical points are x=8,2,4


But given that x0
Our critical point is x=4 only
f(x) changes sign from negative to positive as x crosses 4 from left to right. So x=4 is the point of local minima.
Now compare value of function at x=4 with value of function at the boundary points. So,
f(4)=10×156=25
f(0)=33
when x, f
Hence, the minimum value of f(x) is 25

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