let PQRS be a quadrilateral . if M and N are the midpoints of the sides PQ and RS respectively then prove that PS + QR=2MN-
here  - means bar
 

Dear Student, 

Let P = x,y Q =a,bR= m,nS =u,vSo PS+ QR = m-ai+n-bj+ u-xi+v-yj=m-a+u-xi+n+v-y-bj......1Now M = x+a2,y+b2 and N = m+u2,n+v2 MN =m+u2-x+a2 i+n+v2-y+b2j=12m+u-a-xi+12n+v-y-bjSo 2MN = m-a+u-xi+n+v-y-bj= PS+ QRUsing equation 1. Regards


 

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