# Inverse function help

aquitaine
Ok, I decided to review basic algebra since I haven't done anything with it in like, forever. I came across an inverse function problem that I can't get the right answer.

the equation is:

y = cuberoot(x+sqrt(1+x^2)) + cuberoot(x-sqrt(1+x^2))

I tried replacing X with Y, and solving for Y
and getting rid of the cube roots by cubing both sides
X^3 = y + sqrt(1+y^2) + y - sqrt(1+y^2)
simplifying a bit (the square roots go away)
x^3 = 2y
so
y = (1/2)x^3

Yet the book I'm using says the answer is y=(1/2)(3x+x^3)

What did I do wrong?

Homework Helper
(a+ b)3 is NOT a3+ b3

It is a3+ 3a2b+ 3ab2+ b3

Gold Member
Ok, I decided to review basic algebra since I haven't done anything with it in like, forever. I came across an inverse function problem that I can't get the right answer.

the equation is:

y = cuberoot(x+sqrt(1+x^2)) + cuberoot(x-sqrt(1+x^2))

I tried replacing X with Y, and solving for Y
and getting rid of the cube roots by cubing both sides
X^3 = y + sqrt(1+y^2) + y - sqrt(1+y^2)
simplifying a bit (the square roots go away)
x^3 = 2y
so
y = (1/2)x^3

Yet the book I'm using says the answer is y=(1/2)(3x+x^3)

What did I do wrong?

Your equation is a tad confusing, you have y = $$\sqrt{x+sqrt(1+x^2)}$$ + $$\sqrt{x-sqrt(1+x^2)}$$

then you cube both sides and switch y's and x's to get $$x^{3}$$ = y+$$\sqrt(1+y^2)$$ + y-$$\sqrt(1+y^2)$$

Which is wrong. Now if your equation was y = $$\sqrt{x+\sqrt(1+x^2)+x-\sqrt(1+x^2)}$$ this method would be correct. However, it would have been simplified easily before you even cube both sides; In this case it would be in the form y = $$\sqrt{2x}$$ for the roots cancel automatically.

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