# How to remove roots from an equation?

The question here is how (if it is even possible) to remove the square root terms and transform the following equation to a polynomial with one unknown $$x$$. The coefficients $$a$$, $$b$$, $$c$$, and $$d$$ are known and also $$r$$.

$$a \sqrt{x} + b \sqrt{x} \sqrt{r^2-x^2} + c \sqrt{r^2-x^2} + d = 0$$

• – Dave L. Renfro Feb 26 at 21:14
• As @DaveLRenfro alludes, removing roots is only possible if we can exactly factor the expression which is set to equal zero. – hardmath Feb 26 at 21:28

Yes it is possible. Take $$\color{red}{x=r \cos t, r>0}$$. Because $$r^2=(-r)^2$$. So, we can accept $$r>0$$ . Then we have,

$$a \sqrt {r}\sqrt {\cos t}+br\sqrt {\cos t}|\sin t|+cr|\sin t|+d=0$$

$$\sqrt {\cos t}\left(a\sqrt r+br |\sin t|\right)=-d-cr|\sin t|$$

$$\cos^2 t\left (a\sqrt r+br |\sin t|\right)^4=\left(-d-cr|\sin t|\right)^4$$

$$(1-|\sin t|^2)\left(a\sqrt r+br |\sin t|\right)^4=\left(d+cr|\sin t|\right)^4$$

Then $$\color{red}{|\sin t|=y}$$, you get

$$(1-y^2) (a\sqrt r+bry)^4-(d+cr y)^4=0.$$

Finally, you get $$6$$ degree polynomial respect to $$y.$$

I believe you can take from here.

• @Bashar you are welcome. :) – lone student Feb 26 at 22:49

Well the $$r-x^2$$ screams that they want a trig substitution as lone student's answer suggest.

But you can always remove roots but bringing terms over and squaring.

$$a \sqrt{x} + b \sqrt{x} \sqrt{r^2-x^2} + c \sqrt{r^2-x^2} + d = 0$$

$$\sqrt{x}(a + b\sqrt{r^2-x^2}) = - d - c\sqrt{r^2-x^2}$$

$$x(a^2 + b(r^2-x^2) + 2ab\sqrt{r^2-x^2})) = d^2 + c^2(r^2 - x^2) - 2cd\sqrt{r^2-x^2}$$

$$x(a^2 + b(r^2-x^2))- d^2 - c^2(r^2-x^2) = -(2cd+2abx)\sqrt{r^2 -x^2}$$

$$(x(a^2 + b(r^2-x^2))+ d^2 + c^2(r^2-x^2))^2 = (2cd+2abx)^2(r^2-x^2)$$ so

$$(x(a^2 + b(r^2-x^2))+ d^2 + c^2(r^2-x^2))^2 -(2cd+2abx)^2(r^2-x^2) =0$$

Which is a $$6$$th degree polynomial

which 1) Answers exactly what you asked and 2) makes things much, much, much worse.

• Make things much worse? – Some Guy Feb 26 at 22:30
• I guess he means "now the expression is a mess because it has a lot more terms. It is, indeed. Polynomial things are not necessarily easier to solve! – Andrea Marino Feb 26 at 22:35
• Seems obviously worse to me. Admittedly it only has $6$ terms when we are done simplying an having coefficients expressed in terms of $a,b,c,d,r$ but we are no closer to solving and $\frac 23$ of our solutions will be extraneous. – fleablood Feb 26 at 22:44
• Where you have the coefficient $b$ multiplying $(r^2-x^2)$ after the first squaring, it should be a $b^2$. Also, the $-2cd$ should be $+2cd$ – Barry Cipra Feb 26 at 23:19