<s>[INST] The plane $2x + 4y + 8z = 0$ is perpendicular to the plane $ax + by + 2z = 0$ if and only if $\frac{1}{8}a + \frac{1}{2}b + 1 = 0.$ Therefore the coefficients $a$ and $b$ cannot be both positive. In fact, $a$ and $b$ cannot both be negative.[/INST]