LaTeX rendering is not working for certain equations

[ghost]

When @@a \ne 0@@, there are two solutions to @@\(ax^2 + bx + c = 0\)@@ and they are
$$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$

$$F_{n}=F_{n-1}+F_{n-2}$$

The last one does not render. The first sentence works.

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Screenshot

[gscacco]

The problem is the underscore symbol. The following works:

$$F_{n} = F_{n-1}+F_{n-2}$$

[benfb]

@gscacco Thanks for covering this!