>>38
|a↑+x*b↑+y*c↑|^2
= (1+2x-y)^2 + (-1+2x-y)^2 + (-3+x)^2
= 11-6x+9x^2-8xy+2y^2
= 2(y-2x)^2 +x^2 -6x+11
= 2(y-2x)^2 +(x-3)^2 + 2

y-2x=0
x-3=0
すなわち,
x=3
y=6
のとき最小値√2