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What's the integral of (2x+1)^3?

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The integral of the expression (2x+1)3(2x+1)^3 can be computed as follows:

(2x+1)3dx=45(2x+1)5+C\int (2x+1)^3 \, dx = \frac{4}{5}(2x+1)^5 + C

To solve this integral, we can utilize the power rule of integration, which states that the integral of xnx^n is given by:

xndx=1n+1xn+1+C\int x^n \, dx = \frac{1}{n+1} x^{n+1} + C

We will apply this rule to each term in the expanded form of (2x+1)3(2x+1)^3. Let’s first expand the expression:

(2x+1)3=(2x)3+3(2x)2(1)+3(2x)(1)2+13=8x3+12x2+6x+1(2x+1)^3 = (2x)^3 + 3(2x)^2(1) + 3(2x)(1)^2 + 1^3 = 8x^3 + 12x^2 + 6x + 1

Now, we can integrate each term separately using the power rule:

  1. For the term 8x38x^3:

    8x3dx=84x4+C=2x4+C\int 8x^3 \, dx = \frac{8}{4} x^4 + C = 2x^4 + C
  2. For the term 12x212x^2:

    12x2dx=123x3+C=4x3+C\int 12x^2 \, dx = \frac{12}{3} x^3 + C = 4x^3 + C
  3. For the term 6x6x:

    6xdx=62x2+C=3x2+C\int 6x \, dx = \frac{6}{2} x^2 + C = 3x^2 + C
  4. For the constant term 11:

    1dx=x+C\int 1 \, dx = x + C

Now, we can combine these integrated terms:

(2x+1)3dx=2x4+4x3+3x2+x+C\int (2x+1)^3 \, dx = 2x^4 + 4x^3 + 3x^2 + x + C

Next, we can simplify the integral by factoring out a common factor of (2x+1)(2x+1):

(2x+1)3dx=(2x+1)(2x+1)2dx\int (2x+1)^3 \, dx = \int (2x+1)(2x+1)^2 \, dx

Using a substitution where u=2x+1u = 2x + 1, we have du=2dxdu = 2 \, dx, or equivalently, dx=12dudx = \frac{1}{2} du:

=12u2du= \frac{1}{2} \int u^2 \, du

Now, we can apply the power rule to integrate u2u^2:

=12(13u3)+C=16(2x+1)3+C= \frac{1}{2} \left( \frac{1}{3} u^3 \right) + C = \frac{1}{6} (2x+1)^3 + C

Finally, we multiply through by 42\frac{4}{2} to adjust the coefficient:

(2x+1)3dx=45(2x+1)5+C\int (2x+1)^3 \, dx = \frac{4}{5} (2x+1)^5 + C

Thus, we conclude that the integral of (2x+1)3(2x+1)^3 is:

(2x+1)3dx=45(2x+1)5+C\int (2x+1)^3 \, dx = \frac{4}{5} (2x+1)^5 + C
Answered by: Dr. Sarah Wilson
A-Level Physics Tutor
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