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2014-07-16T10:14:42+03:00
A=AB*BC * sin <B/2

sin<B=120

sin<120=sin(90+30)= cos<30 =√3/2

A=4*5 *√3/2/2

A=2*5 *√3/2

A=5*√3

A=5√3
1 2 1