将原式分子分母同除以ab得：{2/b-7-2/a}/{1/b-3+1/b}= {2(1/a+1/b)-7} / {(1/a+1/b)-3}= {2*4-7}/}4-3}= 1/1= 1
1/a+1/b=4(a+b)/ab=4∴a+b=4ab(a-3ab+b)/(2a-7ab+2b)=[(a+b)-3ab]/[2(a+b)-7ab]=(4ab-3ab)/(8ab-7ab)=1
