Question

Use logarithmic differentiation to find the derivative of y.
y = x(x + 8) (x + 9)

Answer 1

y'=3x^2+34x+72

Explanation:

I see this has no answer and it might be too late.

It's asking us to find dy/dx, after taking log of both sides and apply any useful properties of log.

Take log of both sides:

log(y)=log[x(x+8)(x+9)]

Apply product rule of logarithms on right:

log(y)=log(x)+log(x+8)+log(x+9)

Now differentiate both sides:

y'/y=1/x+1/(x+8)+1/(x+9)

Multiply both sides by y:

y'=y(1/x+1/(x+8)+1/(x+9))

Replace y with x(x+8)(x+9) -> this is from the given equation:

y'=x(x+8)(x+9)(1/x+1/(x+8)+1/(x+9))

Distribute:

y'=(x+8)(x+9)+x(x+9)+x(x+8)

Multiply/distribute some more:

y'=x^2+17x+72+x^2+9x+x^2+8x

Combine like terms:

y'=3x^2+34x+72