...若a=(a1,a2,a3),b=(b1,b2,b3)则a1/b1=a2/b2=a3/b3是a//b的()

问题描述:

...若a=(a1,a2,a3),b=(b1,b2,b3)则a1/b1=a2/b2=a3/b3是a//b的()

∵︱a︱=5,︱b︱=6 a*b=|a|*|b|*cosa=30∴cosa=1∴向量a与向量b同向共线设b=ka=(k*a1,k*a2,k*a3)=(b1,b2,b3),(k>0)又∵︱a︱=5,︱b︱=6∴(a1)^2+(a2)^2+(a3)^2=25 k^2*((a1)^2+(a2)^2+(a3)^2)=36∴k^2=36/25 即k=6/5∴(a1+a2+a3)/(b1+b2+b3)=(a1+a2+a3)/k*(a1+a2+a3)=1/k=5/6.