# How to prove there are no solutions to the equation $a^2-4ab+b^2=0$ if $a$ and $b$ are real numbers and $b \neq 0$?

I am trying to prove $a^2-4ab+b^2=0$ has no solutions for all real numbers $a$ and $b$ and $b \neq 0$

My attempt:

We know that $a^2 \geq 0$ and $b^2 > 0$ since $b \neq 0$. So then $a^2 + b^2 >0$. Now I'm stuck as I'm not sure how to show that $a^2 + b^2 = 4ab$ has no solutions given the above conditions. A little assistance would be greatly appreciated.

• Try looking for counterexamples. You can't prove a false statement. – David K Dec 12 '16 at 17:53
• Maybe you're looking for a lack of rational/integer solutions? Then solving the quadratic (basically) does the trick... – Steven Stadnicki Dec 12 '16 at 17:57
• but it has integer solutions for some real values of $a$ and $b$. – Jorge Fernández Hidalgo Dec 12 '16 at 17:59

## 6 Answers

This has real solutions.

Solve the quadratic equation for $a$ and you find

$a = (2 \pm \sqrt{3}) b$

This isn't true. Consider $a = 1,$ and $b = 2 + \sqrt{3}$.

Then $a^{2} -4ab + b^{2} = 1 -4(2 + \sqrt{3}) + (2 + \sqrt{3})^{2} = 1 - 8 -4\sqrt{3} + 4 +4\sqrt{3} +3 = 0$.

Perhaps the question meant to ask over the integers instead of the reals?

Complete the square and write it as:

$$(a-2b)^2 = 3 b^2 \\ a-2b = \pm \sqrt{3}\,b \\ a = (2 \pm \sqrt{3})b$$

since $$b\ne 0$$ we can divide by $b$ and we get $$\frac{a^2}{b^2}-4\frac{a}{b}+1=0$$ Setting $$\frac{a}{b}=t$$ you will got a quadratic equation. Can you proceed?

notice that $a^2-4ab+b^2$ is a polynomial of degree $2$ with variable $a$.

So we can solve it with the quadratic formula.

You get $\frac{4b\pm\sqrt{16b^2-4b^2}}{2}=(2 \pm \sqrt{3}) b$, which in fact is always a real number.

See if you can find all integer solutions to $$a^2 - 4 a b + b^2 = 1.$$ You might as well stick with $a,b \geq 0.$ If we had $a \geq 1, b \leq -1,$ or $a \leq -1, b \geq 1,$ we would then have $a^2 - 4ab + b^2 \geq 6.$ So, if both nonzero, either both positive or both negative.