Пусть 1 доска весит х кг, а 1 кирпич весит у кг
{7х+3у=71
{3х-2у=14
умножаем второе уравнение на 1,5
4,5х-3у=21
складываем 2 уравнения
7х+3у+4,5х-3у=71+21
11,5х=92
х=92:11,5
х=8
7*8+3у=71
3у=71-56
3у=15
у=15:3
у=5
Ответ: 8 кг весит доска, 5 кг весит кирпич
А)-17-56=-73
б)19,07-15,9=3,17
в)0*(-0,03)=0
г)-19/40*(-35/57)=7/8*3=7/24
д)-1:(-2,2)=10/22=5/11
е)5/24:16/100=-5*100/24*16=-50/384=-1 116/384=-1 29/96
<span>1, 2, 3, 6, 53, 106, 159, 318 </span>
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Ч.Т.Д.
P.S. В конце использована формула сокращенного умножения сумма кубов.