Solve 7/n+3+3/n^2-9

$$\frac { 7 } { n + 3 } + \frac { 3 } { n ^ { 2 } - 9 }$$

Evaluate

$$\frac{7n-18}{n^{2}-9}$$

Short Solution Steps

Factor n^{2}-9.

$$\frac{7}{n+3}+\frac{3}{\left(n-3\right)\left(n+3\right)}$$

To add or subtract expressions, expand them to make their denominators the same. Least common multiple of n+3 and \left(n-3\right)\left(n+3\right) is \left(n-3\right)\left(n+3\right). Multiply \frac{7}{n+3} times \frac{n-3}{n-3}.

$$\frac{7\left(n-3\right)}{\left(n-3\right)\left(n+3\right)}+\frac{3}{\left(n-3\right)\left(n+3\right)}$$

Since \frac{7\left(n-3\right)}{\left(n-3\right)\left(n+3\right)} and \frac{3}{\left(n-3\right)\left(n+3\right)} have the same denominator, add them by adding their numerators.

$$\frac{7\left(n-3\right)+3}{\left(n-3\right)\left(n+3\right)}$$

Do the multiplications in 7\left(n-3\right)+3.

$$\frac{7n-21+3}{\left(n-3\right)\left(n+3\right)}$$

Combine like terms in 7n-21+3.

$$\frac{7n-18}{\left(n-3\right)\left(n+3\right)}$$

Expand \left(n-3\right)\left(n+3\right).

$$\frac{7n-18}{n^{2}-9}$$

Differentiate w.r.t. n

$$\frac{-7n^{2}+36n-63}{\left(n^{2}-9\right)^{2}}$$

Steps Using Derivative Rule for Quotient

Factor n^{2}-9.

$$\frac{\mathrm{d}}{\mathrm{d}n}(\frac{7}{n+3}+\frac{3}{\left(n-3\right)\left(n+3\right)})$$

To add or subtract expressions, expand them to make their denominators the same. Least common multiple of n+3 and \left(n-3\right)\left(n+3\right) is \left(n-3\right)\left(n+3\right). Multiply \frac{7}{n+3} times \frac{n-3}{n-3}.

$$\frac{\mathrm{d}}{\mathrm{d}n}(\frac{7\left(n-3\right)}{\left(n-3\right)\left(n+3\right)}+\frac{3}{\left(n-3\right)\left(n+3\right)})$$

Since \frac{7\left(n-3\right)}{\left(n-3\right)\left(n+3\right)} and \frac{3}{\left(n-3\right)\left(n+3\right)} have the same denominator, add them by adding their numerators.

$$\frac{\mathrm{d}}{\mathrm{d}n}(\frac{7\left(n-3\right)+3}{\left(n-3\right)\left(n+3\right)})$$

Do the multiplications in 7\left(n-3\right)+3.

$$\frac{\mathrm{d}}{\mathrm{d}n}(\frac{7n-21+3}{\left(n-3\right)\left(n+3\right)})$$

Combine like terms in 7n-21+3.

$$\frac{\mathrm{d}}{\mathrm{d}n}(\frac{7n-18}{\left(n-3\right)\left(n+3\right)})$$

Consider \left(n-3\right)\left(n+3\right). Multiplication can be transformed into difference of squares using the rule: \left(a-b\right)\left(a+b\right)=a^{2}-b^{2}. Square 3.

$$\frac{\mathrm{d}}{\mathrm{d}n}(\frac{7n-18}{n^{2}-9})$$

For any two differentiable functions, the derivative of the quotient of two functions is the denominator times the derivative of the numerator minus the numerator times the derivative of the denominator, all divided by the denominator squared.

$$\frac{\left(n^{2}-9\right)\frac{\mathrm{d}}{\mathrm{d}n}(7n^{1}-18)-\left(7n^{1}-18\right)\frac{\mathrm{d}}{\mathrm{d}n}(n^{2}-9)}{\left(n^{2}-9\right)^{2}}$$

The derivative of a polynomial is the sum of the derivatives of its terms. The derivative of a constant term is 0. The derivative of ax^{n} is nax^{n-1}.

$$\frac{\left(n^{2}-9\right)\times 7n^{1-1}-\left(7n^{1}-18\right)\times 2n^{2-1}}{\left(n^{2}-9\right)^{2}}$$

Do the arithmetic.

$$\frac{\left(n^{2}-9\right)\times 7n^{0}-\left(7n^{1}-18\right)\times 2n^{1}}{\left(n^{2}-9\right)^{2}}$$

Expand using distributive property.

$$\frac{n^{2}\times 7n^{0}-9\times 7n^{0}-\left(7n^{1}\times 2n^{1}-18\times 2n^{1}\right)}{\left(n^{2}-9\right)^{2}}$$

To multiply powers of the same base, add their exponents.

$$\frac{7n^{2}-9\times 7n^{0}-\left(7\times 2n^{1+1}-18\times 2n^{1}\right)}{\left(n^{2}-9\right)^{2}}$$

Do the arithmetic.

$$\frac{7n^{2}-63n^{0}-\left(14n^{2}-36n^{1}\right)}{\left(n^{2}-9\right)^{2}}$$

Remove unnecessary parentheses.

$$\frac{7n^{2}-63n^{0}-14n^{2}-\left(-36n^{1}\right)}{\left(n^{2}-9\right)^{2}}$$

Combine like terms.

$$\frac{\left(7-14\right)n^{2}-63n^{0}-\left(-36n^{1}\right)}{\left(n^{2}-9\right)^{2}}$$

Subtract 14 from 7.

$$\frac{-7n^{2}-63n^{0}-\left(-36n^{1}\right)}{\left(n^{2}-9\right)^{2}}$$

For any term t, t^{1}=t.

$$\frac{-7n^{2}-63n^{0}-\left(-36n\right)}{\left(n^{2}-9\right)^{2}}$$

For any term t except 0, t^{0}=1.

$$\frac{-7n^{2}-63-\left(-36n\right)}{\left(n^{2}-9\right)^{2}}$$