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Quadratic Equation and Inequalities question

2024 · 5 Apr · Shift 2 · Q58
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  5. /2024 · 5 Apr · Shift 2 · Q58

Quadratic Equation and Inequalities question

2024 · 5 Apr · Shift 2 · Q58

JEE MainMathematicsQuadratic Equation and InequalitiesNumerical+4 / −1
The number of real solutions of the equation x∣x+5∣+2∣x+7∣−2=0x|x+5|+2|x+7|-2=0x∣x+5∣+2∣x+7∣−2=0 is ‾\underline{\hspace{2cm}}​.
Numerical answer
View written solutionFree

Correct answer: 3

We need to find the number of real solutions of x∣x+5∣+2∣x+7∣−2=0.x|x+5|+2|x+7|-2=0.x∣x+5∣+2∣x+7∣−2=0.

We solve it by breaking into cases according to the absolute values.


1. Critical points

The expressions inside modulus are:

  • x+5=0⇒x=−5x+5=0 \Rightarrow x=-5x+5=0⇒x=−5
  • x+7=0⇒x=−7x+7=0 \Rightarrow x=-7x+7=0⇒x=−7

So we consider three intervals:

  1. x≥−5x\ge -5x≥−5
  2. −7≤x<−5-7\le x< -5−7≤x<−5
  3. x<−7x< -7x<−7

2. Case I: x≥−5x\ge -5x≥−5

Then ∣x+5∣=x+5,∣x+7∣=x+7.|x+5|=x+5,\qquad |x+7|=x+7.∣x+5∣=x+5,∣x+7∣=x+7. So the equation becomes x(x+5)+2(x+7)−2=0.x(x+5)+2(x+7)-2=0.x(x+5)+2(x+7)−2=0. Simplifying, x2+5x+2x+14−2=0x^2+5x+2x+14-2=0x2+5x+2x+14−2=0 x2+7x+12=0.x^2+7x+12=0.x2+7x+12=0. Factorizing, (x+3)(x+4)=0.(x+3)(x+4)=0.(x+3)(x+4)=0. Thus, x=−3,−4.x=-3,-4.x=−3,−4. Both satisfy x≥−5x\ge -5x≥−5, so both are valid.

So from this case, we get 2 solutions.


3. Case II: −7≤x<−5-7\le x< -5−7≤x<−5

Then ∣x+5∣=−(x+5),∣x+7∣=x+7.|x+5|=-(x+5),\qquad |x+7|=x+7.∣x+5∣=−(x+5),∣x+7∣=x+7. So the equation becomes x[−(x+5)]+2(x+7)−2=0.x[-(x+5)] +2(x+7)-2=0.x[−(x+5)]+2(x+7)−2=0. That is, −x2−5x+2x+14−2=0-x^2-5x+2x+14-2=0−x2−5x+2x+14−2=0 −x2−3x+12=0.-x^2-3x+12=0.−x2−3x+12=0. Multiply by −1-1−1: x2+3x−12=0.x^2+3x-12=0.x2+3x−12=0. Using the quadratic formula, x=−3±9+482=−3±572.x=\frac{-3\pm\sqrt{9+48}}{2}=\frac{-3\pm\sqrt{57}}{2}.x=2−3±9+48​​=2−3±57​​. Now check which root lies in [−7,−5)[-7,-5)[−7,−5):

  • −3+572>0\frac{-3+\sqrt{57}}{2}>02−3+57​​>0, not valid.
  • −3−572≈−3−7.552≈−5.27,\frac{-3-\sqrt{57}}{2}\approx \frac{-3-7.55}{2}\approx -5.27,2−3−57​​≈2−3−7.55​≈−5.27, which lies in [−7,−5)[-7,-5)[−7,−5), so it is valid.

So from this case, we get 1 solution.


4. Case III: x<−7x< -7x<−7

Then ∣x+5∣=−(x+5),∣x+7∣=−(x+7).|x+5|=-(x+5),\qquad |x+7|=-(x+7).∣x+5∣=−(x+5),∣x+7∣=−(x+7). So the equation becomes x[−(x+5)]+2[−(x+7)]−2=0.x[-(x+5)] +2[-(x+7)]-2=0.x[−(x+5)]+2[−(x+7)]−2=0. Thus, −x2−5x−2x−14−2=0-x^2-5x-2x-14-2=0−x2−5x−2x−14−2=0 −x2−7x−16=0.-x^2-7x-16=0.−x2−7x−16=0. Multiply by −1-1−1: x2+7x+16=0.x^2+7x+16=0.x2+7x+16=0. Discriminant: Δ=72−4⋅1⋅16=49−64=−15<0.\Delta=7^2-4\cdot 1\cdot 16=49-64=-15<0.Δ=72−4⋅1⋅16=49−64=−15<0. So there is no real solution in this case.


5. Total number of real solutions

Adding all valid solutions: 2+1+0=3.2+1+0=3.2+1+0=3.

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


6. Comparison with stored answer

Stored correct answer: 333

Our derived answer is also 333, so it agrees.

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