555

What would you like to do with 555? It can mean a few things. Here are the main interpretations you might encounter in exams:

• Number theory (math): 555 = 3 × 5 × 37. Divisors: 1, 3, 5, 15, 37, 111, 185, 555. Euler totient φ(555) = 288. In Roman numerals, 555 = DLV.
• Electronics: 555 timer IC. Used in astable/monostable multivibrator circuits. Formulas (simplified):
  • Astable frequency: f ≈ 1.44 / ((R_A + 2R_B) C)
  • High time: t_H ≈ 0.693 (R_A + R_B) C
  • Low time: t_L ≈ 0.693 R_B C
• Thai internet slang: 555 represents laughter (because 5 is pronounced “ha” in Thai).
• Other contexts: fictional phone numbers starting with 555; etc.

Which one do you want to review or practice? If you share a specific topic or a problem, I’ll tailor the explanation and work through it with you.