Q.

If x is real, the maximum and minimum values of expression x2+14x+9x2+2x+3 will be

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a

5,-4

b

-4,-5

c

4,-5

d

-4,5

answer is A.

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Detailed Solution

Given expression is x2+14x+9x2+2x+3=y (say)

 x2+14x+9=y(x2+2x+3)  (1-y)x2+2x(7-y)+3(3-y)=0

Since above equation is a quadratic equation in 'x'.

Since 'x' is Real

 D0

Here D=2(7-y)2-4(1-y)3(3-y)

 D=4(49+y2-14y)-12(3-y-3y+y2)  D=-8y2-8y+160

Now  D0

 -8y2-8y+1600  y2+y-200  (y-4)(y+5)0  y[-5,4]

Therefore the minimum and maximum values of the expression will be -5  and 4. Hence option (a) is correct.

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