If x ∈ R, then minimum and maximum limit of   are

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RPSC 2nd Grade Mathematics (Held on 4th July 2019) Official Paper
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  1. 1/4, 2
  2. 1, 2
  3. 1/2, 2
  4. 3/2, 2

Answer (Detailed Solution Below)

Option 3 : 1/2, 2
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RPSC Senior Grade II (Paper I): Full Test 1
100 Qs. 200 Marks 120 Mins

Detailed Solution

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The correct option is ( 3 ).

Concept : 

Maximum and Minimum Values of the Quadratic Expression :

Let's suppose y = ax2 + bx + c ,

when determining the maximum and minimum values of ax2 + bx + c.

suppose x is real ,then the descriminant of equation 

ax2 + bx + c - y = 0

⇒ b2 - 4a(c - y) = 0

Calculation :

Let,

y = 

 

yx2 + 2xy + 9y = x2 - 2x + 9

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

Now,

D = 0

b2 - 4ac = 0

⇒ ( 2y + 2 ) - 4( y - 1 ) 9 ( y - 1 ) = 0

4y2 + 4 + 8y - 36y2 - 36 + 72y = 0

32y2 - 80y + 32 = 0

2y2 - 5y + 2 = 0

2y ( y - 2 ) - 1 ( y - 2 ) = 0

( y - 2 ) ( 2y - 1 ) = 0

⇒ y = 2 and , 

Thus,

maximum value of the function = 2

minimum value of the function = 

Hence, The correct option is ( 3 ).

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