1.(1+a)(1+b)-(1+a+b)
=1+a+b+ab-1-a-b
=ab
因为a=1/3,b=1/7
所以原式=ab=1/3×1/7=1/21
2.-x+3xy—y/3x—5xy+3y
=(3y-x)+(3xy-5xy)-y/3x
=3y-x-2xy-y/3x
=9xy/3x-6x²y/3x-y/3x
=(9xy-6x²y-y)/3x
=-y(6x²-9x+1)/3x