依题意:
S1=AC×AC/2=AC²/2
S2=BC×BC/2=BC²/2
S3=AB×AB/2=AB²/2
则S1+S2=(AC²+BC²)/2
因∠ACB=90
则AC²+BC²=AB²
则S1+S2=AB²/2
则S1+S2=S3