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Q.

If ax2+bxc for all positive x, where a,b>0, then

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a

27ab24c3

b

27ab2<4c3

c

4ab227c3

d

None of these

answer is A.

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Detailed Solution

Given function is ax2+bxc

Assume the given function as  y=ax2+bxc

Differentiate w.r.t x

dydx=2ax-bx2 dydx=2ax3-bx2

Differentiate again w.r.t x

d2ydx2=2a+2bx3 >0

Equate dydx=0

x=b2a13

Observe that d2ydx2x=b2a13 is positive.

f(x) has minimum value at x=b2a13

Substitute the value of x in the given  function.

                      ax2+bxc ab2a23+bb2a13c                 ab2a+bcb2a13                            3b2cb2a13

Cube on both sides,

           27b38c3b2a 27b38×2abc3            27ab24c3

Therefore, the solution of the given function is 27b24c3.

Hence, the correct answer is option 1.

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