In the FE handbook, on page 25, there's an equation for a right circular cone defined as the cross-sectional area of the cone varies with the square of the distance from the apex. The equation reads:
$$A_x:A_b=x^2:h^2$$
What is the colon operator? Lindeburg solves this for $A_b$ by showing:
$$A_b=\frac{h^2A_x}{x^2}$$
it is just what you use for ratios , $a:b$ means "a is to b".you can read more here.
Alternatively you can write it as :
$$\dfrac{A_x}{A_b}=\dfrac{x^2}{h^2}\implies A_b =\dfrac{h^2\cdot A_x}{x^2}$$