3a. Show that 2⁰ + 2¹ + 2² + 2³ = 2⁴ − 1.

Evaluate both sides: 2⁰ + 2¹ + 2² + 2³ = 1 + 2 + 4 + 8 = 15, while 2⁴ − 1 = 16 − 1 = 15. The sides are equal.