點解西樂要pentatonic scale////>.?

To answer this question, you need to refer to chord & scale theory.

You can see that pentatonic scales are well-fitted to most of the chords in improvisation.

Just take a simple example of C-cycle..
for I-IV-V, we have C,F,G

If you play C pentatnic scales on C, u will hv, C,D,E,G,A which includes the triad notes of C,E & G.

a pentatonic scale is not like the normal major or minor scales (d,r,m,f,s,l,t) but a little simpler. the pentatonic scale consists of 5 notes only. for example, the minor pentatonic has only d,r,m,s,l. same as "play_jazz_piano" says, just adding sth

if ure saying pentatonic scale..it is widely used in guitar solos (combining blues) and some other culture might have this kind of tune as well...