Арифметическая прогрессия
a1 = 20
d = 4
<span>n = 16
</span><span>S = (a1 +an)*n/2 = (2a1 + (n - 1)d)*n/2 = (2*20 + (16 - 1)*4)*16/2 = 800
</span><span>
</span>
<span>4(3х-2)-4(х-2)=2
12х-8-4х+8=2
8х=2
х=2
</span>
7y = 14
y=2
5x+4y=3
5x+8=3
5x=-5
x = -1
Ответ: (-1;2)