Calculus 2, Fall 2016

Prelude to substitution

  1. Does every elementary function have an elementary antiderivative?

  2. Indefinite integrals are _______________, so to evaluate indefinite integrals we need to be able to undo ________________.

  3. Is it possible to use graph transformations to simplify area problems?

  4. Is it possible to use a change of variables to solve integrals?

  5. Is it possible to undo the chain rule for derivatives and simplify evaluating all integrals?

An area problem

Simplify the area problem by finding another parabola that has the same area under it as the given parabola. Draw it!

Another area problem

Find the area between the function \(f(t) = (2t-1)^2\) and the \(t\)-axis for \(0 \leq t \leq 3\). Graph the function and then simplify the area problem by finding another parabola that has the same area under it as the given parabola.

Another area problem

These graphs show that for the change of coordinates \(u = 2t-1,\) \[\int_{t=0}^{t=3} (2t-1)^2 \, dt = \int_{u=-1}^{u=5} \frac{u^2}{2} \, du.\]

Substitution

Let \(F\) and \(g\) be differentiable functions where the range of \(g\) is an interval contained in the domain of \(F\). If we write \(u = g(x)\), then \[\begin{array}{rcl} \int F'(g(x)) g'(x) \, dx & = & \int F(u) \, du \\ & = & F(u) + C \\ & = & F(g(x)) + C \end{array}\] and \[\begin{array}{rcl} \int_{x=a}^{x=b} F'(g(x)) g'(x) \, dx & = & \int_{u=g(a)}^{u=g(b)} F'(u) \, du \\ & = & F(g(b)) - F(g(a)). \end{array}\]

Basic examples

  1. \(\displaystyle \int 3x^2 \cos\left(x^3\right) \, dx\)

  2. \(\displaystyle \int t^3 \sqrt{t^4 + 5} \, dt\)

  3. \(\displaystyle \int 3 (e^{2x} - 4x^7) \, dx\)

  4. \(\displaystyle \int \frac{y^4}{y^5+1} \, dy\)

  5. \(\displaystyle \int \frac{1}{9+x^2} \, dx\)

More challenging examples

  1. \(\displaystyle \int_0^4 w \sqrt{4-w} \, dw\)

  2. \(\displaystyle \int \frac{x+1}{x^2+2x+19} \, dx\)

  3. \(\displaystyle \int \frac{ e^{\sqrt{x}} }{ \sqrt{x} } \, dx\)

  4. \(\displaystyle \int \sqrt{1 + \sqrt{x}} \, dx\)

  5. \(\displaystyle \int \frac{x + 2 \sqrt{x-1}}{ 2x \sqrt{x-1}} \, dx\)

Definite integrals

  1. \(\int_0^{\pi/4} \tan(\theta) \, d\theta\)

  2. \(\displaystyle \int_{1}^{8} \frac{ e^{\sqrt[3]{x}} }{ \sqrt[3]{x^2} } \, dx\)

  1. \(\int_{0}^{\pi} \cos(x+\pi) \, dx\)

  2. Oil is leaking out of Enbridge Pipeline 5 under the Mackinac Bridge into Lake Michigan and Lake Huron at a rate of \(r(t) = 50 e^{-0.02t}\) thousand liters per minute. How many liters leak out during the first hour?

Thinky-thinky questions

  1. Suppose \(\int_0^2 g(t) \, dt = 5\).

    1. Find \(\int_0^4 g(t/2) \, dt\)

    2. Find \(\int_0^2 g(2-x) \, dx\)

  2. Calculate exactly:

    1. \(\int_{-\pi}^{\pi} \cos^2(\theta) \sin(\theta) \, d\theta\).

    2. Find the exact area under the curve \(y = \cos^2(t)\sin(t)\) on \(0 \leq t \leq \pi\)

Thinky-thinky questions

Consider \(\int \cos(\theta) \sin(\theta) \, d\theta\).

  1. Evaluate the integral.

  2. You probably used either \(u = \cos(\theta)\) or \(u = \sin(\theta)\). Use the other substitution to solve the integral.

  3. Use a trig identity such as \(\cos(2\theta) = \cos^2(\theta) - \sin^2(\theta)\) or \(\sin(2\theta) = 2\sin(\theta)\cos(\theta)\) to solve the integral.

  4. You should have found three expressions for \(\int \cos(\theta) \sin(\theta) \, d\theta\). Are they really different? Are they all correct? Explain.