(a+b)a-(a-b)b=a^2+b^2
a^2+ab-ab+b^2=a^2+b^2
a^2+ab-ab+b^2-a^2-b^2=0
0=0
А1= 3*1+1 = 4
а14= 3*14+1 = 43
S14=(a1+an)*n / 2 = (4 + 43) * 14 / 2 = 329
10,5 - (2х - 3) = 4,2; 10,5 - 2х + 3 = 4,2; 13,5 - 2х = 4,2; 2х = 13,5 - 4,2; 2х = 9,3; х = 9,3 : 2; х = 4,65.
437-(x-83)=20
х-83=437-20
х-83=417
х=417+83
<span>х=500</span>