两边平方： (a-b)^2 = a^2 - 2ab + b^2 = 2 - 2ab=9/4ab=-1/8(a+b)^2 = a^2 + 2ab + b^2=2-1/4 =7/4
a-b=3/2(a-b)^2=9/4a^2+b^2-2ab=9/42-2ab=9/42ab=2-9/42ab=-1/4(a+b)^2=a^2+b^2+2ab=2-1/4=7/4
7/4
