SAT官方数学习题 8月19日

Correct Answer: C

• Here's Why:

  Recall that x² – y² can be factored as (x + y)(x – y). Since x + y = 3 and x – y = 5, their product, x² – y², is (3)(5), or 15.

  Another method of solution is to solve the two
  equations x + y = 3 and x – y = 5 simultaneously to get x = 4 and y = –1. Then x² – y² is 4² – (–1)² = 16 – 1 = 15.

• Difficulty: Medium
• Question Type: Standard Multiple Choice