Find position of centre of mass of four particle system, which are at the vertices of parallelogram, as shown in figure.

Explanation

From figure
`DC=bsin theta,OD=bcos theta`
`x_(CM)=(m_(1)x_(1)+m_(2)x_(2)+m_(3)x_(3)+m_(4)x_(4))/(m_(1)+m_(2)+m_(3)+m_(4))`
`x_(CM)=([a+bcos theta)]/2`, similarly `y_(CM)=(bsin theta)/2`
`:. [x_(CM),y_(CM)]=[(a+bcostheta)/2,(bsintheta)/2]`