A2=8 a1+d=8
a10=-40 a1+9d=-40
a1+9d-a1-d=-48 8d=-48 d=-6 a1-6=8 a1=14
s18=(2a1+(n-1)d)*n/2=(2*14-17*6)*18/2=(28-102)*9=-666
1000x⁹+100x⁶y²+10/3x³y⁴+1/27y⁶ = 1/27 (27000x⁹+2700x⁶y²+90x³y⁴+y⁶) = 1/27 (30x³+y²)³
(а+1)(а+2)(а-3)-а(а-4)+5 = (a²+3a+2)(a-3)-a²+4a+5 = a³-3a²+3a²-9a+2a-6-a²+4a+5 = a³<span>-a²</span>-3a-1
(x-2y)(x+2y)-x²=x²-4y²-x²=-4y²=-4·2,5²=-4·6,25=-25