Use a^2 − b^2 factorization with decimals: Evaluate 13.065^2 − 3.065^2 by rewriting it as (a − b)(a + b) and computing quickly.

Difficulty: Easy

Correct Answer: 161.3

Explanation:


Introduction / Context:
The classic identity a^2 − b^2 = (a − b)(a + b) is a powerful shortcut, especially with decimals. It replaces squaring and subtracting with two additions/subtractions and one multiplication, greatly reducing error likelihood and saving time.


Given Data / Assumptions:

  • a = 13.065, b = 3.065.
  • Compute a^2 − b^2.


Concept / Approach:
Apply a^2 − b^2 = (a − b)(a + b). The numbers are chosen so that a − b is an integer, making the product particularly simple to evaluate accurately.


Step-by-Step Solution:

a − b = 13.065 − 3.065 = 10.000.a + b = 13.065 + 3.065 = 16.130.Therefore a^2 − b^2 = (a − b)(a + b) = 10.000 × 16.130 = 161.3.


Verification / Alternative check:

Rough check: 13^2 − 3^2 = 169 − 9 = 160, near 161.3, consistent with decimal offsets.


Why Other Options Are Wrong:

  • 159.5 and 141.6: Result from mis-adding a + b or misplacing decimals.
  • 100: Confuses a^2 − b^2 with (a − b)^2 or only takes a − b.


Common Pitfalls:

  • Losing decimal places during addition.
  • Attempting the full squaring, increasing arithmetic risk.


Final Answer:

161.3

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