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MTH 102 MTH 102 Past Questions Objective Question

Question

Find dy/dx If y = xtan x
Options
A dy/dx = -x<sup>tan x</sup> (tan x / x - sec&sup2; x ln x)
B dy/dx = -x<sup>tan x</sup> (tan x / x + sec&sup2; x ln x)
C dy/dx = x<sup>tan x</sup> (tan x / x - sec&sup2; x ln x)
D dy/dx = x<sup>tan x</sup> (tan x / x + sec&sup2; x ln x)
Correct Answer
Correct Answer

Option D is the correct answer.

Detailed Explanation

This is a function of the form f(x)g(x), so we use logarithmic differentiation.Let y = xtan x.Take the natural logarithm of both sides: ln y = tan x * ln x.Now, differentiate both sides with respect to x, using implicit differentiation on the left and the product rule on the right:(1/y) dy/dx = (d/dx tan x) * ln x + tan x * (d/dx ln x)(1/y) dy/dx = (sec² x) * ln x + tan x * (1/x)Multiply both sides by y:dy/dx = y * (sec² x ln x + tan x / x)Substitute y = xtan x back into the equation:dy/dx = xtan x (sec² x ln x + tan x / x)This matches option D.

Hint

For a function raised to a function (e.g., f(x)^g(x)), logarithmic differentiation is usually the easiest method. Take the natural logarithm of both sides first.

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