1)627+163=790 2)937-637=300 3)63.000х790=49.770.000 4)49.770.000х300=14931.000000 5)14931.000000х90.000=1343790.000000000
x + 2y = 6
-x + y = 3
y = 3 + x
x + 2(3 + x) = 6
x + 6 + 2x = 6
3x + 6 = 6
3x = 0
x = 0
y = 3
Или так:
- x = 3 - y
x = -(3-y)
x = -3 + y
-3 + y + 2y = 6
-3 + 3y = 6
3y = 6 + 3
3y = 9
Оба варианта правильны.