# Calculate the double integral of the expression 6x/(1 + xy) dA, where R = [0, 6] × [0, 1].

This question aims to find the double integral of the given expression over a given range in $x-axi$ and $y-axis$.

This question is based on the concept of integration, particularly double integrals. The integration is used to find the surface area of two-dimensional regions and the volume of three-dimensional objects.

We have the following double integral expression given as:

$\iint_{R}^{} (\dfrac{6x}{1 + xy}) dA$

The range is given as:

$R = {(x, y): 0 \le x \le 6, 0 \le y \le 1}$

The following formulas are used to solve the question.

$\int x^n dx = \dfrac{x^{n + 1}}{n + 1} + C$

$\int kx dx = k \dfrac{x^2}{2} + C$

$\int \dfrac{1}{\sqrt{x}} dx = \int x^{-\frac{1}{2}} dx$

Hence, we can evaluate the given expression as follows:

$\iint_{R}^{} (\dfrac{6x}{1 + xy}) dA = \int_{0}^{6} \int_{0}^{1} \dfrac{6x}{1 + xy} dy dx$

Based on the variables, we have separated the integrals for the $dx$ and $dy$ as:

$= \int_{0}^{6} 6x dx \int_{0}^{1} (1 + xy)^{-1} dy$

$= \int_{0}^{6} 6x dx \left[ ln(1 +xy) \dfrac{1}{x} \right]_{0}^{1}$

$= \int_{0}^{6} \dfrac{6x}{x} dx \left[ ln(1 +xy) \right]_{0}^{1}$

By inserting the integral values and simplifying the expression as:

$= \int_{0}^{6} 6 dx \left[ln(1 + x) – 0 \right]$

$= 6\int_{0}^{6} ln(1 + x) dx$

$= 6\left[ln(1 + x)(1 + x) – x \right]_{0}^{6}$

By inserting the integral values and simplifying the expression for $dy$ as:

$= 6\left[ln(1 + 6)(1 + 6) – 6 \right]$

$= 42 \times ln(7) – 36$

$= 45.7$

## Numerical Results

The double integral of the given expression is as follows:

$\iint_{R} (\dfrac{6x}{1 + xy}) dA = 45.7$

## Example

Calculate the double derivative of the expression given below.

$\int_{1}^{2}\int_{4}^{9}\dfrac{3 + 5y}{\sqrt{x}} dx dy$

Simplifying the expression:

$= \int_{1}^{2}\int_{4}^{9}(3 + 5y)x^{-\frac{1}{2}} dx dy$

Then, based on the variables, we have separated the integrals for the $dx$ and $dy$ as:

$=\int_{1}^{2}(3 + 5y)dy \int_{4}^{9}x^{-\frac{1}{2}} dx$

$= \int_{1}^{2}(3 + 5y)dy \left[ \frac{x^{- \frac{1}{2} + 1}}{\frac{-1}{2} + 1} \right]_{4}^{9}$

$= \int_{1}^{2}(3 + 5y)dy \left[ \frac{x^{\frac{1}{2}}}{\frac{1}{2}} \right]_{4}^{9}$

We insert the integral values and simplify the expression for $dx$ as:

$= \int_{1}^{2}(3 + 5y)dy \left[ 2(9^{\frac{1}{2}} – 4^{\frac{1}{2}}) \right]$

$= \int_{1}^{2}(3 + 5y)dy \left[ 2(3 – 2) \right]$

$= 2\int_{1}^{2}(3 + 5y)dy$

$= 2\left[3y + \frac{5y^2}{2} \right]_{1}^{2}$

We insert the integral values and simplify the expression for $dy$ as follows:

$= 2\left[ 3(2 – 1) + \frac{5}{2}(2^2 – 1^2) \right]$

$= 2\left[ 3 + 5 \times 1.5 \right]$

$= 2(10.5)$

$= 21$

Hence, we have the final value as:

$\int_{1}^{2}\int_{4}^{9}\dfrac{3 + 5y}{\sqrt{x}} dx dy = 21$