- #1
Chsoviz0716
- 13
- 0
Say we are solving an indefinite integral ∫x√(2x+1) dx.
According to the textbook, the solution goes like this.
Let u = 2x+1. Then x = (u-1)/2.
Since √(2x+1) dx = (1/2)√u du,
x√(2x+1) dx = [(u-1)/2] * (1/2)√u du.
∫x√(2x+1) dx = ∫[(u-1)/2] * (1/2)√u du. <= What justifies this??
The rest is just trivial calculation.
The only theorem I could rely on here to solve this problem was,
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If u=g(x) is a differentiable function whose range is an interval I, and f is continuous no I, then
∫f(g(x))g'(x) dx = ∫f(u) du.
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Which is so called the substitution rule.
How can this theorem justify the process of the solution I wrote above?
According to the textbook, the solution goes like this.
Let u = 2x+1. Then x = (u-1)/2.
Since √(2x+1) dx = (1/2)√u du,
x√(2x+1) dx = [(u-1)/2] * (1/2)√u du.
∫x√(2x+1) dx = ∫[(u-1)/2] * (1/2)√u du. <= What justifies this??
The rest is just trivial calculation.
The only theorem I could rely on here to solve this problem was,
-------------------------------------------------------------------
If u=g(x) is a differentiable function whose range is an interval I, and f is continuous no I, then
∫f(g(x))g'(x) dx = ∫f(u) du.
--------------------------------------------------------------------
Which is so called the substitution rule.
How can this theorem justify the process of the solution I wrote above?