Given : PQR is right triangle at Q. M and N are mid points of QR and PQ respectively.
Now, In △NQR,
RN2=QR2+NQ2 (By Pythagoras theorem)
RN2=QR2+(PQ2)2 (N is mid point of PQ)
4RN2=4QR2+PQ2...(I)
Now, In △PQM,
PM2=PQ2+QM2 (By Pythagoras theorem)
PM2=PQ2+(QR2)2 (M is mid point of QR)
4PM2=4PQ2+QR2...(II)
Adding I and II,
4(RN2+PM2)=4PQ2+4QR2+QR2+PQ2
4(RN2+PM2)=5(PQ2+QR2)
In △PQR,
PR2=PQ2+QR2
Hence, 4(RN2+PM2)=5PR2