1) 900-b^2=30^2-b^2=(30-b)(30 +b)
2)0,81y^4-49a^2b^2=(0,9y^2)^2-(7ab)^2=(0,9y^2-7ab)(0,9y^2+7ab)
3)81y^3-y=y*(81y^2-1)
1)(9-m)(m+9)=9^2-m^2= -m^2+81
2)(6-3k)^2=36-36k+9k^2=9k^2-36k+36
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Cos(8П/6)=V3/2
8Пx/6=+-П/6+2Пк;к€Z
X=+-1/8+3k/2;k€Z
X=1/8+3к/2;к€Z