10. 三棱柱ABC−A1B1C1ABC-A_{1}B_{1}C_{1}ABC−A1B1C1中，MMM、NNN分别是A1BA_{1}BA1B、B1C1B_{1}C_{1}B1C1上的点，且BM=2A1MBM=2A_{1}MBM=2A1M，C1N=2B1NC_{1}N=2B_{1}NC1N=2B1N．设AB→=a→\overrightarrow{AB}=\overrightarrow{a}AB=a，AC→=b→\overrightarrow{AC}=\overrightarrow{b}AC=b，AA1→=c→\overrightarrow{A{A}_{1}}=\overrightarrow{c}AA1=c．（Ⅰ）试用a→,b→,c→\overrightarrow{a},\overrightarrow{b},\overrightarrow{c}a,b,c表示向量MN→\overrightarrow{MN}MN；（Ⅱ）若∠BAC=90∘\angle BAC=90\circ∠BAC=90∘，∠BAA1=∠CAA1=60∘\angle BAA_{1}=\angle CAA_{1}=60\circ∠BAA1=∠CAA1=60∘，AB=AC=AA1=1AB=AC=AA_{1}=1AB=AC=AA1=1，求MNMNMN的长．{width="1.9479166666666667in" height="1.583333333
{width="1.9479166666666667in" height="1.583333333