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What is the LCM of \(x^4+x^2y^2 + y^4\), \(x^3y+y^4\) and \(x^4y^2 - x^3 y^3\)?

This question was previously asked in
CDS 2 2025 Maths Question Paper (14-Sep-2025)
The correct answer is
\(x^3y^2 (x^6-y^6)\)

Factoring the Algebraic Expressions

To find the Least Common Multiple (LCM) of the given expressions, we first need to factor each expression completely.

Expression 1: \(x^4+x^2y^2 + y^4\)

This expression can be factored by adding and subtracting \(x^2y^2\) to complete the square:

  • \(x^4+x^2y^2 + y^4 = (x^4 + 2x^2y^2 + y^4) - x^2y^2\)
  • \(= (x^2+y^2)^2 - (xy)^2\)
  • Using the difference of squares formula (\(a^2 - b^2 = (a-b)(a+b)\)):
  • \(= (x^2+y^2 - xy)(x^2+y^2 + xy)\)
  • Rearranging the terms within the factors:
  • \(= (x^2 - xy + y^2)(x^2 + xy + y^2)\)

Expression 2: \(x^3y+y^4\)

Factor out the common term \(y\):

  • \(x^3y+y^4 = y(x^3+y^3)\)
  • Using the sum of cubes formula (\(a^3+b^3 = (a+b)(a^2-ab+b^2)\)):
  • \(= y(x+y)(x^2 - xy + y^2)\)

Expression 3: \(x^4y^2 - x^3 y^3\)

Factor out the common term \(x^3y^2\):

  • \(x^4y^2 - x^3 y^3 = x^3y^2(x - y)\)

Finding the LCM

The LCM is found by taking the highest power of each unique factor present in the factorizations of the expressions.

The unique factors are:

  • \(x^3\) (highest power from Expression 3)
  • \(y^2\) (highest power from Expression 3)
  • \((x-y)\) (from Expression 3)
  • \((x+y)\) (from Expression 2)
  • \((x^2 - xy + y^2)\) (present in Expressions 1 and 2, highest power is 1)
  • \((x^2 + xy + y^2)\) (from Expression 1)

Multiplying these factors together gives the LCM:

LCM \(= x^3 \cdot y^2 \cdot (x-y) \cdot (x+y) \cdot (x^2 - xy + y^2) \cdot (x^2 + xy + y^2)\)

Simplifying the LCM Expression

We can simplify the product of factors using algebraic identities:

  • Recall the difference of cubes: \(a^3 - b^3 = (a-b)(a^2+ab+b^2)\).
  • Recall the sum of cubes: \(a^3 + b^3 = (a+b)(a^2-ab+b^2)\).
  • Also, recall the difference of sixth powers: \(a^6 - b^6 = (a^3)^2 - (b^3)^2 = (a^3 - b^3)(a^3 + b^3)\).
  • Substituting the factored forms of cubes:
  • \(x^6 - y^6 = [(x-y)(x^2+xy+y^2)] \cdot [(x+y)(x^2-xy+y^2)]\)
  • Rearranging the terms, we see that \((x-y)(x+y)(x^2-xy+y^2)(x^2+xy+y^2)\) is exactly the product of the polynomial factors we found.

Therefore, the LCM can be written as:

LCM \(= x^3 y^2 (x^6 - y^6)\)

Conclusion

By factoring each polynomial and identifying the highest power of all unique factors, we determined the LCM. The LCM of \(x^4+x^2y^2 + y^4\), \(x^3y+y^4\), and \(x^4y^2 - x^3 y^3\) is \(x^3y^2 (x^6 - y^6)\).

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Important Questions from LCM and HCF

  1. The greatest three-digit number which is divisible by 14, 28, and 42 is:

  2. What is the greatest number that will divide 209 and 347 leaving remainder 5 and 7 respectively?

  3. A number is three times another number and their HCF is 8. What is the sum of the squares of the numbers?

  4. The HCF of 2091, 3485 and 4879 is x. The sum of the digits of x is:

  5. If three numbers are in ratio of 3 : 5 : 7 and their LCM is 2415, what is the difference between the second number and the first number?

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