You need the formula for geometric mean for right triangles as it applies to the altitude. For us, that formula will be [tex] \frac{BD}{AD}=\frac{CD}{BD} [/tex] where BD is your altitude. The formula then, filled in accordingly, would be [tex] \frac{10}{x}=\frac{4x}{10} [/tex]. We cross multiply to get that [tex] 4x^2=100 [/tex]. Divide both sides by 4 to get [tex] x^2=25 [/tex], so x = 5. Choice C is the one you want.
