Б) 4а^2-(а+3б)^2=4а^2-а^2+6аб+9б^2=3а^2+6аб+9б^2
<span>-4(х-10)=12(х-1)
-4х+40=12х-12
-4х-12х=-12-40
-16х=-52
х=-52:(-16)
х=3.25
</span><span>-х+2=х-6
-х-х=х-6
-2х=-8
х=-8:(-2)
х=4</span>
An= 7+8n
S20-?
a1= 7+8×1=15
a2=7+8×2=23
d=23-15=8
S20=(2×15+(20-1)×8)×20/2=(30+153)×10=182×10=1820
Ответ: S20=1820