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Sin(51)cos(21)−cos(51)sin(21)=1/2(Sin(51 -21) + Sin(51+21)) − 1/2(Sin(51+21) - Sin(51+21))=1/2(Sin(30) + Sin(72)) − 1/2(Sin(72) - Sin(30))=1/2(Sin(30) + Sin(72) − Sin(72) + Sin(30))=1/2(Sin(30) + Sin(30))= 1/2(1/2 + 1/2)=1/2
3х+2х=45
5х=45
Х=45:5=9
2*9=18 грушевых деревьев
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(a+5) (a-2)+(a+4) (a-5) =
= а² + 5а - 2а - 10 + а² + 4а - 5а - 20 =
= 2а² + 2а - 30=2(а² + а - 15)