Daily Calculus - Post one Problem and its Solution
Posted: Mon May 27, 2019 11:56 am
Calculus is a natural witchcraft to many problems in life, it is highly applied to solve a wide range of problems in sciences, engineering, mathematics, humanities, economics, and many other areas. In this Daily Calculus thread, I encourage anyone to post at least a problem and its solution to help others learn faster.
Problem 1.
Evaluate
\begin{align}
\int \frac{dx}{\sqrt{1 - x^2}}.
\end{align}
Solution:
Let's apply trigonometric substitution
\begin{align}
x = {\rm sin}u, \implies {\rm d} x = {\rm cos}u{\rm d}u.
\end{align}
Then
\begin{align}
\begin{split}
\int \frac{dx}{\sqrt{1 - x^2}} &\quad = \int \frac{{\rm cos}u{\rm d}u}{\sqrt{1 - {\rm sin}^{2}u}}\\
\\
&\quad = \int \frac{{\rm cos}u{\rm d}u}{\sqrt{{\rm cos}^{2}u}} \ ({\rm since } \ \ {\rm cos}^{2}u + {\rm sin}^{2}u = 1)\\
\\
&\quad = \int \frac{{\rm cos}u{\rm d}u}{{\rm cos}u}\\
\\
&\quad = \int{\rm d}u = u + c = {\rm sin}^{-1}x + c
\end{split}
\end{align}
where \(c\) is a constant.
Problem 2.
Evaluate
\begin{align}
\int\frac{{\rm d}x}{\sqrt{1 - ax^{2}}}.
\end{align}
Solution:
By the analogy of the solution of the integral above,
\begin{align}
\int\frac{{\rm d}x}{\sqrt{1 - ax^{2}}} = \frac{{\sin}^{-1}(\sqrt{a}x)}{\sqrt{a}} + c. \ \ ({\rm Hint}: \ {\rm Make \ substitution} \ \sqrt{a}x = {\rm sin}u.)
\end{align}
Problem 1.
Evaluate
\begin{align}
\int \frac{dx}{\sqrt{1 - x^2}}.
\end{align}
Solution:
Let's apply trigonometric substitution
\begin{align}
x = {\rm sin}u, \implies {\rm d} x = {\rm cos}u{\rm d}u.
\end{align}
Then
\begin{align}
\begin{split}
\int \frac{dx}{\sqrt{1 - x^2}} &\quad = \int \frac{{\rm cos}u{\rm d}u}{\sqrt{1 - {\rm sin}^{2}u}}\\
\\
&\quad = \int \frac{{\rm cos}u{\rm d}u}{\sqrt{{\rm cos}^{2}u}} \ ({\rm since } \ \ {\rm cos}^{2}u + {\rm sin}^{2}u = 1)\\
\\
&\quad = \int \frac{{\rm cos}u{\rm d}u}{{\rm cos}u}\\
\\
&\quad = \int{\rm d}u = u + c = {\rm sin}^{-1}x + c
\end{split}
\end{align}
where \(c\) is a constant.
Problem 2.
Evaluate
\begin{align}
\int\frac{{\rm d}x}{\sqrt{1 - ax^{2}}}.
\end{align}
Solution:
By the analogy of the solution of the integral above,
\begin{align}
\int\frac{{\rm d}x}{\sqrt{1 - ax^{2}}} = \frac{{\sin}^{-1}(\sqrt{a}x)}{\sqrt{a}} + c. \ \ ({\rm Hint}: \ {\rm Make \ substitution} \ \sqrt{a}x = {\rm sin}u.)
\end{align}