|a-2b|^2 = |a|^2 +4|b|^2 -4a.b = |a|^2 +4|b|^2
|a+b|^2 = |a|^2+|b|^2 +2a.b = |a|^2 +|b|^2
丨(a-2b)丨/丨(a+b)丨
=√[(|a|^2 +4|b|^2)/(|a|^2 +|b|^2)]A0B1 C2 D4 有得数的