The equations are in eqnarray and I want to underline only the last line. Is this possible?

Best Answer

The trick is to underline the parts to the left and the right of the & separately. The \mathstruts are used to vertically align the two halves and add extra space before the = and below the denominator. For large structures one can use \vphantom to equalize the depths.

As pointed out by @barbara beeton, the position of the equation number is ambiguous. This can be solved by using the subsidiary environment multlined provided by mathtools inside an equation (first proposed by @egreg). This approach also benefits from the improved spacing of the equation environment.

You'll notice that the equation is left-aligned, although not flush with the left margin. To achieve this, add \setlength{\mathindent}{0pt} to the document preamble as well.

Note that you don't need eqnarray for a single equation. For this, the equation environment is sufficient. However, since the spacing of eqnarray is inferior to the regular math spacing, it might be better for consistency.

## Best Answer

The trick is to underline the parts to the left and the right of the

`&`

separately. The`\mathstrut`

s are used to vertically align the two halves and add extra space before the`=`

and below the denominator. For large structures one can use`\vphantom`

to equalize the depths.