x-10=7y-70
x-7y=-60 (1)
15+x=30+2y
x-2y=15 (2)
(2)-(1)
-2y+7y=15+60
5y=75
y=15
x=15+2y=45
2ab=a+b
两边除以ab
2=a/ab+b/ab
即1/a+1/b=2