Leo realized then that derivatives weren't just abstract symbols; they were the math of motion, growth, and decay. He wasn't just solving for

and didn't panic. He took the derivative of the "outside" (cosine) and multiplied it by the derivative of the "inside" (4).

By the time he reached the section, the frustration that usually tightened his chest began to loosen. He saw

Leo sat at his desk, the blue light of the website glowing like a beacon in his dim room. The header "Derivatives Practice Questions" stared back at him. For weeks, the concept of a derivative—the instantaneous rate of change—had felt less like math and more like a riddle. He clicked the first question: