Openai/690f817c-4724-800b-a047-fe00351b6d9c

Explaining integrals for derivatives

Generated: 2025-11-08 22:18:26

Prompt: Explain, to someone who knows how only derivatives work, how integrals work and what the 2 numbers next to it mean

ChatGPT: Alright — here’s how to think about integrals in a way that connects to derivatives.

1. Derivatives recap (the starting point)

You already know that a derivative tells you how fast something is changing — it’s the slope of a function at any point. For example, if f(x)f(x)f(x) is position, then f′(x)f'(x)f′(x) is velocity.

2. Integrals are the reverse idea

If a derivative tells you the rate of change, an integral tells you the total accumulation of that change. It’s like asking:

“If I know the speed of a car at every moment, how far did it travel overall?”

That total distance comes from adding up all the little bits of movement — each tiny piece of distance = speed × a tiny bit of time.

Mathematically, that “adding up” process is the integral.

So the integral of f(x)f(x)f(x) gives you the total area under its curve — or the accumulated value of f(x)f(x)f(x) — across some interval.

3. The two numbers (the bounds)

When you see something like

∫abf(x) dx\int_a^b f(x)\,dx∫ab​f(x)dx the numbers a and b mean:

• Start at x=ax = ax=a
• Stop at x=bx = bx=b

So this symbol represents the total accumulation (or area) from aaa to bbb.

Visually:

• f(x)f(x)f(x) = height of the curve
• dxdxdx = an infinitely thin width
• f(x) dxf(x)\,dxf(x)dx = a tiny rectangle’s area
• ∫ab\int_a^b∫ab​ = add all those rectangles from x=ax=ax=a to x=bx=bx=b

4. The key relationship (Fundamental Theorem of Calculus)

This is what ties derivatives and integrals together:

∫abf′(x) dx=f(b)−f(a)\int_a^b f'(x)\,dx = f(b) - f(a)∫ab​f′(x)dx=f(b)−f(a) That means integrating a derivative just gives you how much the original function changed between the two x-values. So, in a way, integrals undo derivatives, just as derivatives undo integrals.

5. Example

If f′(x)=3x2f'(x) = 3x^2f′(x)=3x2, then

∫133x2 dx=f(3)−f(1)\int_1^3 3x^2\,dx = f(3) - f(1)∫13​3x2dx=f(3)−f(1) Since f(x)=x3f(x) = x^3f(x)=x3, that’s 27−1=2627 - 1 = 2627−1=26. So the total accumulated change between x=1x=1x=1 and x=3x=3x=3 is 26.

Would you like me to include a simple real-world analogy (like water filling a tank) to make the accumulation idea more intuitive?

Prompt: How would one solve this

ChatGPT: Read the integrand carefully:

∫−22 ⁣(x3cos⁡ ⁣x2+12)4−x2 dx.\int_{-2}^{2}\!\Big(x^3\cos\!\tfrac{x}{2}+\tfrac12\Big)\sqrt{4-x^2}\,dx.∫−22​(x3cos2x​+21​)4−x2​dx. Two quick observations make this easy.

1. Odd/even split. * 4−x2\sqrt{4-x^2}4−x2​ is an even function (symmetric). * x3x^3x3 is odd and cos⁡(x/2)\cos(x/2)cos(x/2) is even, so x3cos⁡(x/2)x^3\cos(x/2)x3cos(x/2) is odd. * Odd × even = odd, so the whole x3cos⁡(x/2)4−x2x^3\cos(x/2)\sqrt{4-x^2}x3cos(x/2)4−x2​ piece is an odd integrand. The integral of any odd function over [−a,a][-a,a][−a,a] is 000. So that term contributes 000.
2. Constant term. The remaining part is 12∫−224−x2 dx.\tfrac12\int_{-2}^{2}\sqrt{4-x^2}\,dx.21​∫−22​4−x2​dx. ∫−224−x2 dx\displaystyle\int_{-2}^{2}\sqrt{4-x^2}\,dx∫−22​4−x2​dx is the area under the upper half of the circle x2+y2=4x^2+y^2=4x2+y2=4, i.e. a semicircle of radius 222. Its area is 12πr2=12π(22)=2π\tfrac12\pi r^2=\tfrac12\pi(2^2)=2\pi21​πr2=21​π(22)=2π. Multiply by 12\tfrac1221​ to get 12⋅2π=π.\tfrac12\cdot 2\pi=\pi.21​⋅2π=π.

So the integral equals π\boxed{\pi}π​ (≈ 3.14159). If the sign asked for the “first digits,” that would be 3.143.143.14.

(If you prefer an algebraic evaluation instead of the geometric one, use the trig substitution x=2sin⁡θx=2\sin\thetax=2sinθ to turn ∫4−x2 dx\int\sqrt{4-x^2}\,dx∫4−x2​dx into a straightforward integral that evaluates to 2π2\pi2π.)

Prompt: Wait, the dx is also in the square root

ChatGPT: Good eye — that’s an important reading question. Short answer: the dxdxdx is not inside the square root — it’s the differential, which always sits outside the radical in standard notation. So the integral is

∫−22(x3cos⁡x2+12)4−x2  dx,\int_{-2}^{2}\Big(x^{3}\cos\frac{x}{2}+\tfrac12\Big)\sqrt{4-x^{2}}\;dx,∫−22​(x3cos2x​+21​)4−x2​dx, not something like 4−x2dx\sqrt{4-x^{2}dx}4−x2dx​. A couple quick points to make that clear:

• Notationally, the expression under the radical is everything between the radical’s braces or the matching parentheses: here that is 4−x24-x^{2}4−x2. The dxdxdx lives after the integrand and indicates the variable of integration. You’ll often see a small space or a comma before dxdxdx, written as 4−x2 dx\sqrt{4-x^{2}}\,dx4−x2​dx, to signal it’s separate.

• Conceptually, dxdxdx is not a number to be plugged into the radical — it tells you you’re summing (integrating) slices of width dxdxdx. It doesn’t change the function whose area you’re finding.

Because of that reading, the previous work stands: the x3cos⁡(x/2)4−x2x^{3}\cos(x/2)\sqrt{4-x^{2}}x3cos(x/2)4−x2​ part is odd and integrates to 000 on [−2,2][-2,2][−2,2], and the remaining 12∫−224−x2 dx\tfrac12\int_{-2}^{2}\sqrt{4-x^{2}}\,dx21​∫−22​4−x2​dx is 12\tfrac1221​ times the area of a semicircle radius 222, giving π\piπ.

If someone did literally write 4−x2dx\sqrt{4-x^{2}dx}4−x2dx​ (rare and nonstandard), it would be ambiguous / nonsensical in the usual Riemann-integral interpretation — avoid that form.