<span>19*21= (20-1) (20+1) =20^2 -1 = 400- 1 =399</span>
5+6=11
110:10=11
22:2=11
7*3=21
210:10=21
441:21=21
А) - 4,
б)- 3,
в)-1........
( 2 (sin (25+15)/2) * (cos (25-15)/2) ) / ( 2 (sin (25-15)/2)* (cos (25+15)/2))=
=(sin20*cos5) / (sin5*cos20) = tg 20 * ctg 5