若a2+b2=1，c2+d2=1，ad-bc=-1，则ab+cd=______．
∵（a+d）2+（b-c）2
=a2+2ad+d2+b2-2bc+c2
=a2+b2+c2+d2+2（ad-bc），
又∵a2+b2=1，c2+d2=1，ad-bc=-1，
∴（a+d）2+（b-c）2=1+1+2×（-1）=0，即∴（a+d）2+（b-c）2=0，
∴a+d=0，即a=-d，
b-c，即b=c，
∴ab+cd=ab-ba=0；
故答案为：0．
| 根据短文内容，选词填空。 | |
| |
| Alice is from Australia. She 1 to our school yesterday. So our teacher let me 2 her around our school this morning. At about nine I 3 her at the gate. When she came, we walked into our school along the road. I introduced the teaching buildings, office buildings, the library, and the gym to her. When we finished 4 , she said that our school was very big and beautiful, and she liked it very much. She 5 she could study here all the time. |