27.(2014,安徽,理)已知两个不相等的非零向量a,b,两组向量x1,x2,x3,x4,x5和y1,y2,-|||-y3,y4,y5均由2个a和3个b排列而成.记 =overrightarrow ({x)_(1)}cdot overrightarrow ({y)_(1)}+overrightarrow ({x)_(2)}cdot overrightarrow ({y)_(2)}+overrightarrow ({x)_(3)}cdot overrightarrow ({y)_(3)}+overrightarrow ({x)_(4)}cdot overrightarrow ({y)_(4)}+ ·y3+x4·y4+-|||-x5·y5,Smin表示S所有可能取值中的最小值.则下列命题正确的有 __ (填序号).-|||-①S有5个不同的值.-|||-②若→(a) 则Smin与|→a|无关;-|||-③若 overrightarrow (a)ykparallel overrightarrow (b)-|||-∥b则Smin与|b|无关;-|||-④若 |overrightarrow (b)|gt 4|overrightarrow (a)|, 则 _(min)gt 0;-|||-⑤若 |overrightarrow (b)|=2|overrightarrow (a)|, _(min)=8(|overrightarrow {a)|}^2, 则a与b的夹角为 dfrac (pi )(4)