What is the derivative of y=xarcsin(x^2)?

1 Answer
Aug 29, 2015

y^' = (2x^2)/sqrt(1-x^4) + arcsin(x^2)

Explanation:

To make the calculations more interesting, I'll assume that you don't know what the derivative of arcsin(x) is.

You can differentiate this function by using implicit differentiation. Start by isolating arcsin(x^2) on one side

y/x = arcsin(x^2)" "color(blue)((1))

This is equivalent to

sin(y/x) = x^2" "color(blue)((2))

Differentiate both sides with respect to x

d/dx(sin(y/x)) = d/dx(x^2)

cos(y/x) * ((dy)/dx * 1/x - y/x^2) = 2x

Rearrange this to get (dy)/dx isolated on one side

(dy)/dx * cos(y/x) * 1/x - y/x^2 * cos(y/x) = 2x

(dy)/dx * cos(y/x) * 1/x = 2x + y/x^2 * cos(y/x)

(dy)/dx = (2x + y/x^2 * cos(y/x))/(cos(y/x) * 1/x)

(dy)/dx = (2x)/(cos(y/x) * 1/x) + y/x^color(red)(cancel(color(black)(2))) * color(red)(cancel(color(black)(x))) * color(red)(cancel(color(black)(cos(y/x))))/color(red)(cancel(color(black)(cos(y/x))))

(dy)/dx = (2x^2)/cos(y/x) * y/x

Use the trigonometric identity

color(blue)(sin^2x + cos^2x = 1)

To write cos(y/x) as a function of sin(y/x)

cos^2x = 1 - sin^2x

sqrt(cos^2x) = sqrt(1-sin^2x)

cosx = sqrt(1 - sin^2x)

Use this identity and equations color(blue)((1)) and color(blue)((2)) to write

(dy)/dx = (2x^2)/sqrt(1-sin^2(y/x)) + arcsin(x^2)

(dy)/dx = (2x^2)/sqrt(1 - (x^2)^2) + arcsin(x^2)

(dy)/dx = color(green)((2x^2)/sqrt(1-x^4) + arcsin(x^2))