EquationsMCQPYQ May 18Question 1006 of 221
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If the sides of an equilateral triangle are shortened by 3 units, 4 units and 5 units respectively and a right triangle is formed then the side of an equilateral triangle is:

Chapter 2 Diagram

Select Your Answer (Keyboard: A, B, C, D)
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Correct Answer

✅ Option c — 8 units

All Options:

  • A6 units
  • B7 units
  • C8 units
  • D10 units

Detailed Solution & Explanation

Let the side of the original equilateral triangle be x\displaystyle x units.
When the sides are shortened by 3\displaystyle 3 units, 4\displaystyle 4 units, and 5\displaystyle 5 units respectively, the new side lengths are:
x−3,x−4,x−5x - 3, \quad x - 4, \quad x - 5
Since these new side lengths form a right-angled triangle, they must satisfy the Pythagorean theorem. The longest side (hypotenuse) will be the one with the smallest subtracted value, which is x−3\displaystyle x - 3. The other two sides are the legs.
Applying the Pythagorean theorem:
(x−3)2=(x−4)2+(x−5)2(x - 3)^2 = (x - 4)^2 + (x - 5)^2
Expanding both sides:
x2−6x+9=(x2−8x+16)+(x2−10x+25)x^2 - 6x + 9 = (x^2 - 8x + 16) + (x^2 - 10x + 25)
x2−6x+9=2x2−18x+41x^2 - 6x + 9 = 2x^2 - 18x + 41
Rearranging all terms to the right-hand side to form a quadratic equation:
x2−12x+32=0x^2 - 12x + 32 = 0
Factoring the quadratic equation:
(x−8)(x−4)=0(x - 8)(x - 4) = 0
This gives two possible values for x\displaystyle x:
x=8orx=4x = 8 \quad or\displaystyle \text{or} \quad x = 4
Let's evaluate both values:
- If x=4\displaystyle x = 4, the side lengths are 4−3=1\displaystyle 4-3 = 1, 4−4=0\displaystyle 4-4 = 0, and 4−5=−1\displaystyle 4-5 = -1. Since side lengths cannot be zero or negative, x=4\displaystyle x = 4 is discarded.
- If x=8\displaystyle x = 8, the side lengths are 8−3=5\displaystyle 8-3 = 5, 8−4=4\displaystyle 8-4 = 4, and 8−5=3\displaystyle 8-5 = 3. A triangle with sides 3\displaystyle 3, 4\displaystyle 4, and 5\displaystyle 5 is a valid right-angled triangle because 32+42=9+16=25=52\displaystyle 3^2 + 4^2 = 9 + 16 = 25 = 5^2.
Thus, the side of the original equilateral triangle is 8\displaystyle 8 units, which corresponds to Option C. (Note: The raw JSON specifies Option B, which is 7\displaystyle 7 units, but that is mathematically incorrect; Option C is the unique, correct solution).
Hence, **Option C** is the correct answer.

About This Chapter: Equations

Paper

Paper 3: Quantitative Aptitude

Weightage

4-6 Marks

Key Topics

Linear, Quadratic and Cubic Equations

This chapter covers Linear, Quadratic and Cubic Equations and is part of Paper 3: Quantitative Aptitude in the CA Foundation exam.

View Official ICAI Syllabus

Exam Strategy Tip

This topic carries 4-6 Marks weightage. Focus on understanding core concepts rather than memorizing.

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