①在直角三角形ABC中,已知两边长为3和4,则第三边长为5;
②△ABC中,若∠A:∠B:∠C=1:5:6,则△ABC是直角三角形;
③三角形的三边a、b、c满足a2+c2=b2,则△ABC是∠C为直角的直角三角形;
④△ABC中,若 a:b:c=1:2: 3 ,则这个三角形是直角三角形.
其中,正确命题的个数为( )
A man worked in a post office. His job was to handle(处理) all 1 that had unclear addresses. One day he came across a letter which was 2to God. He opened it and it 3,“Dear God,I am an 83yearold woman and I live on a very small pension(养老金).Yesterday someone stole my 4.There was a hundred dollars in it,which was all the money I had 5 until my next pension comes. Next Thursday is Thanksgiving Day,and I have invited my last two friends for 6.Without that money,I have 7 to buy food with. I have no family to 8,and you are my only 9.Can you please help me?” The man was 10,and went around showing the letter to all the other workers. Each of them 11 their wallets and donated a few dollars. By the time he 12 his showing,he had collected 96 dollars. 13then he sent it to the old woman. Thanksgiving Day came and went. The workers 14forgot about it. Then one day,there came another letter from the old lady to God.All the 15gathered around when the letter was16.It read,“Dear God,how can I ever thank you enough for what you did for me?Because of your kindness,I was able to 17 a lovely dinner for my friends. We had a very 18 day,and I told my friends about your19 gift. Thank you very much!” Seeing this,all the workers felt 20 with the nice thing they had done. They looked into each other's eyes and smiled.
用计算器求下列各式的值:
如图,P为⊙O外一点,PA,PB是⊙O的切线,A,B为切点,PA=3,∠P=60°,则图中阴影部分的面积为____ .