In my method book I came across some dominant seventh chords. But the funny thing is that among the four notes in the chord, there's the root, the major third, and the flatted seventh. One of those is repeated. But there's no fifth in this particular chord fingering.
Is it because the root string also makes a relatively strong harmonic of the fifth, so that it can, to an extent, still be heard in the chord even when it's not played? Thanks.
More or less.
The perfect fifth interval does little to define a chord (unless the fifth is altered) so it's often left out - not just in sevenths, but all sorts of chords.
It's especially common for pianists to use a voicing like your seventh... instead of having to reach a flatted seventh interval, they put the b7 in the bass and drop the fifth - that puts everything within the range of a fourth, which is very easy to finger.
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The perfect fifth interval that the chord's fifth forms with the root is so consonant that it adds almost nothing to the harmonic effect of seventh chords and can usually be safely omitted - and, as you pointed out, the root supplies a pure fifth in the form of a strong harmonic anyway - not that its needed.
