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=(2QM)2+NQ2 (M is mid point of QR)
RN2=4QM2+NQ2...(I)
Now, In △PQM,
PM2=PQ2+QM2 (By Pythagoras theorem)
PM2=(2QN)2+QM2 (N is mid point of PQ)
PM2=4QN2+QM2...(II)
Adding I and II,
RN2+PM2=4QM2+QN2+4QN2+QM2
RN2+PM2=5(QM2+QN2)
In △MQN,
MN2=NQ2+QM2 (Pythagoras Theorem)
Hence, RN2+PM2=5MN2