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Find the derivative of cot1(18x+9) \cot^{-1}\left(\frac{1}{8x+9}\right) .

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Q. Find the derivative of cot1(18x+9) \cot^{-1}\left(\frac{1}{8x+9}\right) .
  1. Find Inverse Cotangent Derivative: We need to find the derivative of the inverse cotangent function, which is cot1(18x+9)\cot^{-1}\left(\frac{1}{8x+9}\right). Let's denote u=18x+9u = \frac{1}{8x+9}. The derivative of cot1(u)\cot^{-1}(u) with respect to uu is 11+u2-\frac{1}{1+u^2}. We will use the chain rule to find the derivative with respect to xx.
  2. Derivative of u: First, find the derivative of u=18x+9u = \frac{1}{8x+9} with respect to xx. The derivative of 18x+9\frac{1}{8x+9} is 8(8x+9)2-\frac{8}{(8x+9)^2}, using the power rule and the chain rule.
  3. Apply Chain Rule: Now, apply the chain rule. The derivative of cot1(u)\cot^{-1}(u) with respect to xx is the derivative of cot1(u)\cot^{-1}(u) with respect to uu multiplied by the derivative of uu with respect to xx. This gives us 11+u2(8(8x+9)2)-\frac{1}{1+u^2} \cdot \left(-\frac{8}{(8x+9)^2}\right).
  4. Substitute uu: Substitute uu back into the expression to get the derivative with respect to xx. The derivative of cot1(18x+9)\cot^{-1}\left(\frac{1}{8x+9}\right) with respect to xx is 11+(18x+9)2(8(8x+9)2)-\frac{1}{1+\left(\frac{1}{8x+9}\right)^2} \cdot \left(-\frac{8}{(8x+9)^2}\right).
  5. Simplify Expression: Simplify the expression. The denominator of the first fraction becomes 1+(18x+9)2=(8x+9)2(8x+9)2+1(8x+9)2=((8x+9)2+1)(8x+9)21 + \left(\frac{1}{8x+9}\right)^2 = \frac{(8x+9)^2}{(8x+9)^2} + \frac{1}{(8x+9)^2} = \frac{((8x+9)^2 + 1)}{(8x+9)^2}. The derivative simplifies to 1((8x+9)2+1(8x+9)2)×(8(8x+9)2)-\frac{1}{\left(\frac{(8x+9)^2 + 1}{(8x+9)^2}\right)} \times \left(-\frac{8}{(8x+9)^2}\right).
  6. Combine Fractions: Combine the fractions by multiplying the numerators and denominators. This gives us (1×8)/(((8x+9)2+1)/(8x+9)2×(8x+9)2)(-1 \times -8)/(((8x+9)^2 + 1)/(8x+9)^2 \times (8x+9)^2) which simplifies to 8/((8x+9)2+1)8/((8x+9)^2 + 1).

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