Sigma Percentile
JEE Main 2020 (8 January Shift 2)
LEVELJEE Main

Animated Solution for Mathematics - Quadratic Equations: Let be the set of all real roots of the equation, . Then :

Select Answer:

Visualized Solution

Substitution:

  • Let . Since for all real , we have .
  • The equation becomes: .

Identifying Critical Points

  • The absolute value terms and change sign at and .
  • These are our critical points.
  • We divide the domain into three intervals: , , and .

Case 1:

  • Case 1:
  • Both and are negative.

Solving Case 1

  • Substitute into equation:
  • Expand:
  • Simplify:
  • Root is not in . No solution here.

Case 2:

  • Case 2:
  • Here, but .

Solving Case 2

  • Substitute:
  • Expand:
  • Simplify:
  • Root is not in . No solution here.

Case 3:

  • Case 3:
  • Both and are positive.

Solving Case 3

  • Substitute:
  • Expand:
  • Simplify:
  • Quadratic formula:

Rejecting Case 3

  • Positive root:
  • We assumed .
  • Since , this root is also rejected.
  • All cases yielded no valid roots!

The Graphical Proof

  • Let
  • From the graph, the minimum value occurs at .
  • Since for all , has no real roots.

The JEE Trap (Typo in Original Question)

  • The mathematically correct answer is 0 roots (Empty Set).
  • However, the official answer key states Singleton Set (1 root).
  • Why? The original exam had a typo. The intended equation was:
  • With the minus sign, becomes a valid root in Case 2!

The Sigma Insight: Solution of Quadratic Equations

Solution Diagram

Analyzing the Setup

Welcome, fellow explorer of the mathematical landscape. Today, we are standing before a formidable-looking equation:
At first glance, it might seem like a chaotic mess of exponentials and absolute values. In the world of JEE Advanced, complexity is often just a mask for elegance. Let us peel back that mask together.

The Power of Substitution

The first thing that should catch your eye is the recurring presence of . Whenever you see a repeated structure, think of it as a signal to simplify.
Let us define a new variable, . Because is an exponential function, it is strictly positive for all real . Therefore, our new variable must satisfy the constraint .
With this substitution, our 'monster' equation transforms into a much friendlier polynomial form:

The Anatomy of the Modulus

Think of the modulus function as a mirror that flips negative values to positive. The expressions inside, and , are the 'critical points' where the mirror flips.
Specifically, changes sign at , and changes sign at . These points divide our number line into three distinct territories: , , and .
We must investigate each territory to see if any roots are hiding there.

The Three-Act Play

Act I: The Region
In this region, both and are negative. The modulus bars force them to flip: and .
Substituting these into our equation:
Expanding this, we find , which simplifies to . This is a perfect square: , leading to .
Since our condition for this act was , this root is an imposter. We reject it.
Act II: The Region
Here, is non-negative, but is negative. Thus, and .
Our equation becomes:
Expanding this, we get . The terms collapse beautifully, leaving us with , or .
Since is not in the range , we reject this root as well.
Act III: The Region
In the land of , both expressions are positive. The modulus bars vanish, leaving:
Expanding this, we get , which simplifies to . Using the quadratic formula:
Since must be positive, we take . Because is not greater than , we must reject this root.

The Conclusion

We have searched every region and found no valid roots. Graphically, this means the function never touches the -axis; it sits entirely above it.
The set of real roots is the empty set. Never doubt your math—it is the most reliable compass you have.

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