Update length.tex

Changed  $\verticalvector(6,2,3,1)=$ to  $\verticalvector{6\\2\\3\\1}=$
Update angle.tex

changed \verticalvector(1,2) to \verticalvector{1//2}
Update cauchy-schwarz.tex

some typos changed
Merge pull request #4 from sunayana/master

Update length.tex