1981 AHSME Problems/Problem 29

A solution is available here. Pull up find, and put in "Since x is the principal", and you will arrive at the solution.

It's not super clear, and there's some black stuff over it, but its legible.

The solution in the above file/pdf is the following. I tried my best to match it verbatim, but I had to guess at some things. I also did not do the entire solution like this, just parts where I had to figure out what the words/math was, so this transcribed solution could be wrong and different from the solution in the aforementioned file/pdf.

Anyways:

29. (E) Since $x$ is the principal square root of some quantity, $x\geq0$ . For $x\geq0$ , the given equation is equivalent to $a-\sqrt{a+x}=x^2$ or $a=\sqrt{a+x}+x^2.$ The left member is a constant, the right member is an increasing function of $x$ , and hence the equation has exactly one solution. We write $\begin{align*} \sqrt{a+x}&=a-x^2 \\ \sqrt{a+x}+x&=(a+x)-x^2 \\ &=(\sqrt{a+x}+x)(\sqrt{a+x}-x). \end{align*}$

Since $\sqrt{a+x}+x>0$ , we may divide by it to obtain $1=\sqrt{a+x}-x\quad\text{or}\quad x+1=\sqrt{a+x},$ so $x^2+2x+1=a+x,$ and $x^2+x+1-a=0.$

Therefore $x=\frac{-1\pm\sqrt{4a-3}}{2}$ , and the positive root is $x=\frac{-1+\sqrt{4a-3}}{2}$ , the only solution of the original equation. Therefore, this is also the sum of the real solutions. $\text{OR}$

As above, we derive $a-\sqrt{a+x}=x^2$ , and hence $a-x^2=\sqrt{a+x}$ . Squaring both sides, we find that $a^2-2x^2a+x^4=a+x.$

This is a quartic equation in $x$ , and therefore not easy to solve; but it is only quadratic in $a$ , namely $a^2-(2x^2+a)a+x^4-x=0.$

Solving this by the quadratic formula, we find that $\begin{align*} a&=\frac{1}{2}[2x^2+1+\sqrt{4x^4+4x^2+1-4x^4+4x}] \\ &=x^2+x+1. \end{align*}$ [We took the positive square root since $a>x^2$ ; indeed $a-x^2=\sqrt{a+x}$ .]

Now we have a quadratic equation for $x$ , namely $x^2+x+1-a=0,$ which we solve as in the previous solution.

Note: One might notice that when $a=3$ , the solution of the original equation is $x=1$ . This eliminates all choices except (E).

-- OliverA

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1981 AHSME Problems/Problem 29