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Quadratic Equation and Inequalities question

2023 · 15 Apr · Shift 1 · Q33
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  5. /2023 · 15 Apr · Shift 1 · Q33

Quadratic Equation and Inequalities question

2023 · 15 Apr · Shift 1 · Q33

JEE MainMathematicsQuadratic Equation and InequalitiesMCQ+4 / −1
The number of real roots of the equation x∣x∣−5∣x+2∣+6=0x|x|-5|x+2|+6=0x∣x∣−5∣x+2∣+6=0, is :
  1. A
    4
  2. B
    3
  3. C
    5
  4. D
    6
View written solutionFree

Correct answer: B

  1. We need to solve x∣x∣−5∣x+2∣+6=0x|x|-5|x+2|+6=0x∣x∣−5∣x+2∣+6=0 and count the number of real roots.

  2. Since absolute values are present, split according to the sign changes of xxx and x+2x+2x+2.

The critical points are:

  • x=−2x=-2x=−2
  • x=0x=0x=0

So consider three intervals:

  • x≥0x\ge 0x≥0
  • −2≤x<0-2\le x<0−2≤x<0
  • x<−2x<-2x<−2

Case 1: x≥0x\ge 0x≥0

Then ∣x∣=x,∣x+2∣=x+2|x|=x,\qquad |x+2|=x+2∣x∣=x,∣x+2∣=x+2 So the equation becomes x⋅x−5(x+2)+6=0x\cdot x-5(x+2)+6=0x⋅x−5(x+2)+6=0 x2−5x−10+6=0x^2-5x-10+6=0x2−5x−10+6=0 x2−5x−4=0x^2-5x-4=0x2−5x−4=0

Now solve: x=5±25+162=5±412x=\frac{5\pm\sqrt{25+16}}{2}=\frac{5\pm\sqrt{41}}{2}x=25±25+16​​=25±41​​

Since this case requires x≥0x\ge 0x≥0, only x=5+412x=\frac{5+\sqrt{41}}{2}x=25+41​​ is valid.

So this case gives 1 root.


Case 2: −2≤x<0-2\le x<0−2≤x<0

Then ∣x∣=−x,∣x+2∣=x+2|x|=-x,\qquad |x+2|=x+2∣x∣=−x,∣x+2∣=x+2 So x∣x∣=x(−x)=−x2x|x|=x(-x)=-x^2x∣x∣=x(−x)=−x2

Hence the equation becomes −x2−5(x+2)+6=0-x^2-5(x+2)+6=0−x2−5(x+2)+6=0 −x2−5x−10+6=0-x^2-5x-10+6=0−x2−5x−10+6=0 −x2−5x−4=0-x^2-5x-4=0−x2−5x−4=0 Multiply by −1-1−1: x2+5x+4=0x^2+5x+4=0x2+5x+4=0 (x+1)(x+4)=0 (x+1)(x+4)=0(x+1)(x+4)=0 So the roots are x=−1,−4x=-1,-4x=−1,−4

Now check the interval −2≤x<0-2\le x<0−2≤x<0:

  • x=−1x=-1x=−1 is valid
  • x=−4x=-4x=−4 is not valid

So this case gives 1 root.


Case 3: x<−2x<-2x<−2

Then ∣x∣=−x,∣x+2∣=−(x+2)=−x−2|x|=-x,\qquad |x+2|=-(x+2)=-x-2∣x∣=−x,∣x+2∣=−(x+2)=−x−2 So x∣x∣=x(−x)=−x2x|x|=x(-x)=-x^2x∣x∣=x(−x)=−x2

The equation becomes −x2−5(−x−2)+6=0-x^2-5(-x-2)+6=0−x2−5(−x−2)+6=0 −x2+5x+10+6=0-x^2+5x+10+6=0−x2+5x+10+6=0 −x2+5x+16=0-x^2+5x+16=0−x2+5x+16=0 Multiply by −1-1−1: x2−5x−16=0x^2-5x-16=0x2−5x−16=0

Solve: x=5±25+642=5±892x=\frac{5\pm\sqrt{25+64}}{2}=\frac{5\pm\sqrt{89}}{2}x=25±25+64​​=25±89​​

For this case we need x<−2x<-2x<−2.

  • 5+892>0\frac{5+\sqrt{89}}{2}>025+89​​>0, invalid
  • 5−892\frac{5-\sqrt{89}}{2}25−89​​ is negative. Check if it is less than −2-2−2: since 89>9\sqrt{89}>989​>9, 5−892<5−92=−2\frac{5-\sqrt{89}}{2}<\frac{5-9}{2}=-225−89​​<25−9​=−2 so it is valid.

So this case gives 1 root.


  1. Total number of real roots: 1+1+1=31+1+1=31+1+1=3

Therefore, the number of real roots is 3\boxed{3}3​

So the correct option is B.

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