<span>А) (x²+3)²=(х²-3)(х²+3)+6(х²+3)
</span>(х²-3)(х²+3)+6(х²+3)=(х²+3)(х²-3+6)=(х²+3)(х²+3)=(х²+3)²<span>
Б)(4-х²)²=(4-х²)(4+х²)+2x²(х-4)
</span>(4-х²)(4+х²)+2x²(х-4)=16-x⁴+2x³-8x²
(4-х²)²=16-8x²+x⁴
16-x⁴+2x³-8x²≠16-8x²+x⁴ ⇒
(4-х²)²=(4-х²)(4+х²)+2x²(х-4) не является тождеством
a) a₃+ a₇ + a₁₄ + a₁₈ = 10
a₁ + 2d + a₁ + 6d + a₁ + 13d + a₁ + 17d = 10
4a₁ + 38d = 10 | : 2
2a₁ + 19d = 5
![S_{20}=\frac{2a_{1}+19d }{2}*20=(2a_{1}+19d)*10=5*10=50](https://tex.z-dn.net/?f=S_%7B20%7D%3D%5Cfrac%7B2a_%7B1%7D%2B19d%20%7D%7B2%7D%2A20%3D%282a_%7B1%7D%2B19d%29%2A10%3D5%2A10%3D50)
б) a₆ + a₉ + a₁₂ + a₁₅ = - 16
a₁ + 5d + a₁ + 8d + a₁ + 11d + a₁ + 14d = - 16
4a₁ + 38d = - 16
2a₁ + 19d = - 8
![S_{20}=\frac{2a_{1}+19d }{2}*20=(2a_{1}+19d)*10=-8*10=-80](https://tex.z-dn.net/?f=S_%7B20%7D%3D%5Cfrac%7B2a_%7B1%7D%2B19d%20%7D%7B2%7D%2A20%3D%282a_%7B1%7D%2B19d%29%2A10%3D-8%2A10%3D-80)