As shown in the figure, in △ABC and △PQR, AB=PQ, AC=PR, AD and PS are the center lines of two triangles respectively, and AD=PS.

AD and PS are extended to e and t respectively, so that DE=AD and ST=PS, AE=PT. Connect BE, CE, QT and rt.

In △ADC and △EDB, AD=DE, ∠ADC=∠EDB, DC=DB, so △ ADC △ BDE, AC=BE.

Similarly, PR=QT, so BE=QT.

In △ABE and △PQT, AB=PQ, BE=QT and AE=PT, so △ Abe △ PQT, ∠BAE=∠QPT.

Similarly, ∠CAE=∠RPT, so ∠BAC=∠QPR.

In △ABC and △PQR, AB=PQ, ∠BAC=∠QPR, AC=PR, so △ ABC △ PQR.