4х²-9 - 4х²-8х+1=3х
-8= 11х
х= - 8:11 (восемь одиннадцатых)
A₅+a₇-a₁₂=-9 a₁+4d+a₁+6d-a₁-11d=-9 a₁-d=-9 a₁=d-9
a₃+a₂₀=74 a₁+2d+a₁+19d=74 2a₁+21d=74 2(d-9)+21d=74
2d-18+21d=74
23d=92
d=4 ⇒
a₁=4-9=-5
S₁₆=(2a₁+(n-1)d)*n/2=(2*(-5)+15*4)*16/2=50*8=400.
![\overline {a}=(1,-2,m)\; ,\; \overline{b}=(n,6,3)\\\\\overline {a}||\overline {b}\; \to \; \frac{1}{n}=\frac{-2}{6}=\frac{m}{3}\\\\n=\frac{6}{-2}=-3\\\\m=\frac{-2\cdot 3}{6}=-1\\\\|\overline {b}|=3|\overline{a}|\; ,\; \overline {b}=-3\overline {a}](https://tex.z-dn.net/?f=%5Coverline+%7Ba%7D%3D%281%2C-2%2Cm%29%5C%3B+%2C%5C%3B+%5Coverline%7Bb%7D%3D%28n%2C6%2C3%29%5C%5C%5C%5C%5Coverline+%7Ba%7D%7C%7C%5Coverline+%7Bb%7D%5C%3B+%5Cto+%5C%3B+%5Cfrac%7B1%7D%7Bn%7D%3D%5Cfrac%7B-2%7D%7B6%7D%3D%5Cfrac%7Bm%7D%7B3%7D%5C%5C%5C%5Cn%3D%5Cfrac%7B6%7D%7B-2%7D%3D-3%5C%5C%5C%5Cm%3D%5Cfrac%7B-2%5Ccdot+3%7D%7B6%7D%3D-1%5C%5C%5C%5C%7C%5Coverline+%7Bb%7D%7C%3D3%7C%5Coverline%7Ba%7D%7C%5C%3B+%2C%5C%3B+%5Coverline+%7Bb%7D%3D-3%5Coverline+%7Ba%7D)
Векторы противоположнонаправлены и вектор в в 3 раза длиннее вектора а.