题目

   A long time ago there lived a poor slave whose name was Aesop. His face was white, but very homely. When Aesop was about twenty nine years old ,his master lost a great deal of money and was determined to sell his slaves. To do this, he had to take them to a large city where there was a slave market.  The city was far away, and the slaves must walk the whole distance. A number of bundles(成捆的东西) were made up for them to carry. Some of these bundles contained the things they would need on the road. Choose your bundles, boys, said the master. There is one for each of you. A设a、b、c 是单位向量，且a+b=c，则a与c的夹角为______．