2×2 determinant evaluation: The first row of a determinant is [10, 6] and the second row is [3, 5]. What is the value of this determinant (compute ad − bc)?

Difficulty: Easy

Correct Answer: 32

Explanation:


Introduction / Context:
Determinants summarize key properties of matrices and are used in solving linear systems, computing inverses, and analyzing transformations. For 2×2 matrices, the determinant has a simple closed form: ad − bc. This question reinforces accurate arithmetic and sign handling for a basic case.


Given Data / Assumptions:

  • Matrix rows: [10, 6] and [3, 5].
  • Interpret as a 2×2 matrix [[10, 6], [3, 5]].
  • Use the standard 2×2 determinant formula.


Concept / Approach:
For a 2×2 matrix with entries a, b in the first row and c, d in the second row, the determinant value is ad − bc. Careful multiplication and subtraction yield the final scalar.


Step-by-Step Solution:

Let a = 10, b = 6, c = 3, d = 5.Compute products: ad = 10 * 5 = 50; bc = 6 * 3 = 18.Evaluate determinant: ad − bc = 50 − 18 = 32.Thus, the determinant equals 32.


Verification / Alternative check:
Cross-multiplication visualization: draw diagonals, product of the main diagonal minus product of the other diagonal. The arithmetic again gives 32, confirming the calculation.


Why Other Options Are Wrong:

  • 18 or 50: These are intermediate products, not the required difference.
  • −32: Incorrect sign; ad − bc is positive for these values.


Common Pitfalls:

  • Reversing the subtraction order (bc − ad) and getting the wrong sign.
  • Arithmetic slip in 6 * 3 or 10 * 5.


Final Answer:
32

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