... O 90 + A /2, O a 90 - A /2 ...
87. 13. ? 0
... ) ( a , b ) = - ( b , a ); . ) ( a , b ) + ( b , c ) = ( a , c ); . ... ) ( a , b ) = - ( b , a ); . ) ( a , b ) + ( b , c ) = ( a , c ); . ... ) ( - a , b ) = ( a , b ) + 180. 3. ...
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http://www.mccme.ru/free-books/prasolov/planim/gl13.htm -- 10.3 -- 04.08.2004