A12=a1+(n-1)*d a12=3,1+(12-1)*8,7=98.8
А учебник то какой? Автор и т.д
<span> куб разности <span><span>(<span>a-b</span>)</span>^3</span> = <span><span><span><span>a^3-</span><span><span>3<span>a^2</span></span>b</span></span>+<span><span>3a</span><span>b^2-</span></span></span><span>b^3</span></span> </span><span>.
(a-1)^3= a^3-3a^2+3a-1</span>