An=a1+d(n–1)=7+50d–50=50d–43
50d–43=–385
50d=–342
d=–6,84
S50=(a1+an)/2•50=(7–385)•25=
= –9450
Решаем способом сложения, для начала умножаем одно из уравнений на -2, а второе на 3:
-14x-6y=-66
15x+6y=48
2) x=-18
3)5*(-18) + 2y = 16
-90 + 2y = 16
2y = 106
<span>y = 53 </span>
1. 4+8y +8 = 2y - 10 - 7y +9
12 + 8y = -5y - 1
8y + 5y = -1 -12
13y = -13
y = -13 / 13
y = -1
2. 3x/5 = 6+x/3
9x = 30 + 5x
9x - 5x = 30
4x = 30
x = 30/4
x = 7,5
3. 8x - (7x + 8) = 9
8x - 7x - 8 = 9
8x - 7x = 9 + 8
x = 17