Digital SAT Math

SAT Math: If sin(3x - 15°) = cos(2x + 25°), What Is the Value of x?

2026-09-03 5 min read
• Direct Answer / Key Takeaway

Complementary angle theorem: sin(A) = cos(B) => A + B = 90. (3x - 15) + (2x + 25) = 90 => 5x + 10 = 90 => 5x = 80 => x = 16.

Exact Question Text

If sin(3x - 15°) = cos(2x + 25°) for acute angles, what is the value of x?

1. Step-by-Step Formal Solution

  1. sin(A) = cos(90 - A) => (3x - 15) + (2x + 25) = 90.
  2. 5x + 10 = 90 => 5x = 80 => x = 16.

2. Fast Desmos / Calculator Shortcut

Type (3x - 15) + (2x + 25) = 90 in Desmos => plots vertical line x = 16.


3. Distractor Trap Analysis

  • Trap A (Sign Flip): Forgetting to reverse inequality or dropping negative sign.
  • Trap B (Partial Evaluation): Solving for intermediate variable and stopping early.
  • Trap C (Extreme Assumption): Over-inferring beyond textual evidence.
  • Option Correct: Accurately satisfies all algebraic and logical constraints.

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