ITsJonny
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Friday 23 October 2026 18:00:00 GMT
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![Sine is going UP. Cosine is going DOWN. How is that its derivative? 😭 That first climb had me looking at these two graphs like one of them was lying. The derivative is supposed to tell me how fast sine is going up. So why is cosine getting smaller while sine keeps getting higher? Because the hill is flattening out. You can still be walking uphill while the ground gets less steep. Every step takes you higher, but each step adds a little less height than the last. That's what happens from x = 0 to π/2: sin(x): 0 → 1 cos(x): 1 → 0 Sine keeps rising. Its slope is still POSITIVE, but that positive number is shrinking. Cosine is reporting exactly that. At the top, sine has height 1 and slope 0. Different questions. Different answers. The graph is high, but the ground directly under your feet is flat. Then there's the bit that really messed with my head: a negative slope getting closer to zero. Between π and 3π/2, sine falls toward its minimum. Cosine goes from −1 toward 0. The slope is getting less negative while the original line is STILL FALLING. Think −1, −0.5, −0.1. All negative. All downhill. Just less steep each time. At the bottom, sine is −1 and cosine is 0. Low point, flat ground. Same relationship all over again. The sign tells you whether the graph is going uphill or downhill. The size of the slope tells you how steeply. A derivative getting bigger doesn't automatically mean the original graph is going up. And yes, the familiar rule needs radians: d/dx [sin(x)] = cos(x) For a tiny step h, the change in height is approximately cos(x) × h. Divide that height change by h and you get an average slope. Let h approach zero and you get the exact slope at that point. So when you look at a derivative graph, read its HEIGHT as the original graph's SLOPE at the same x. That's the connection that makes these two waves make sense together. Try pausing at the peak: where is sine, and where is cosine? Now try the bottom. Can you explain both without just quoting the rule? Save this for the next time calculus casually turns sine into cosine and expects you to be fine with it. #Calculus #Trigonometry #MathExplained #LearnOnTikTok #GravityAssist](/video/cover/7700291456581340429.webp)
