MCQMediumJEE 2025Quadratic Equations in Complex Numbers

JEE Mathematics 2025 Question with Solution

The sum of the squares of all the roots of the equation x2+2x34=0x^2 + |2x - 3| - 4 = 0 is:

  • A

    3(22)3(2 - \sqrt{2})

  • B

    6(22)6(2 - \sqrt{2})

  • C

    3(32)3(3 - \sqrt{2})

  • D

    6(32)6(3 - \sqrt{2})

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: x2+2x34=0x^2 + |2x - 3| - 4 = 0

Find: The sum of the squares of all the roots.

Split the equation into cases using the absolute value.

For Case 1, when x32x \geq \frac{3}{2}, we have:

x2+2x34=0x^2 + 2x - 3 - 4 = 0

So,

x2+2x7=0x^2 + 2x - 7 = 0

From the solution, the valid root taken is

x=221x = 2\sqrt{2} - 1

For Case 2, when x<32x < \frac{3}{2}, we have:

x2+32x4=0x^2 + 3 - 2x - 4 = 0

So,

x22x1=0x^2 - 2x - 1 = 0

From the solution, the valid root taken is

x=12x = 1 - \sqrt{2}

Now compute the sum of squares:

(221)2+(12)2\left(2\sqrt{2} - 1\right)^2 + \left(1 - \sqrt{2}\right)^2

Expanding as shown in the solution,

=842+1+122+2= 8 - 4\sqrt{2} + 1 + 1 - 2\sqrt{2} + 2 =1262= 12 - 6\sqrt{2} =6(22)= 6(2 - \sqrt{2})

Therefore, the sum of the squares of all the roots is 6(22)6(2 - \sqrt{2}). The correct option is B.

The solution's also displays "The Correct Option is D", but the worked solution concludes 6(22)6(2 - \sqrt{2}), which matches option B. Following the solution working, B is the defensible answer.

Casewise Absolute Value Analysis

Given: x2+2x34=0x^2 + |2x - 3| - 4 = 0

Find: The sum of the squares of all valid roots.

The expression inside the modulus changes sign at

2x3=0x=322x - 3 = 0 \Rightarrow x = \frac{3}{2}

So the equation must be solved in two intervals.

If x32x \geq \frac{3}{2}, then

2x3=2x3|2x - 3| = 2x - 3

Hence,

x2+2x34=0x^2 + 2x - 3 - 4 = 0 x2+2x7=0x^2 + 2x - 7 = 0

The root retained in the solution is

x=221x = 2\sqrt{2} - 1

which satisfies x32x \geq \frac{3}{2}.

If x<32x < \frac{3}{2}, then

2x3=32x|2x - 3| = 3 - 2x

Hence,

x2+32x4=0x^2 + 3 - 2x - 4 = 0 x22x1=0x^2 - 2x - 1 = 0

The root retained in the solution is

x=12x = 1 - \sqrt{2}

which satisfies x<32x < \frac{3}{2}.

Now add the squares of these roots:

(221)2+(12)2\left(2\sqrt{2} - 1\right)^2 + \left(1 - \sqrt{2}\right)^2 =(842+1)+(122+2)= (8 - 4\sqrt{2} + 1) + (1 - 2\sqrt{2} + 2) =1262= 12 - 6\sqrt{2} =6(22)= 6(2 - \sqrt{2})

So the worked result corresponds to option B.

Common mistakes

  • A common mistake is to treat 2x3|2x - 3| as only 2x32x - 3 for all xx. This is wrong because absolute value is piecewise-defined. Split into the two cases x32x \geq \frac{3}{2} and x<32x < \frac{3}{2} before solving.

  • Students often solve both quadratics completely but forget to check whether each root satisfies the case condition. This is wrong because a root from one branch may be invalid in that interval. Always verify each root against its corresponding inequality.

  • Another mistake is to add the roots instead of adding their squares. The question asks for x12+x22x_1^2 + x_2^2, not x1+x2x_1 + x_2. First identify the valid roots, then square each one and add.

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