Question

determinati suma x+y daca se stie ca x-y=9 si 2x+y=21?

Answer 1

$x+y=? \\ \\ x-y=9\\2x+y=21 Adunam \ expresiile\\ ------\\ 3x \ / \ 30 \\ x=30:3 \\ \fbox{x=10} \\ \\10-y=9\\\fbox{y=1} \\ \\ \boxed{\boxed{x+y=10+1=11}}$

Answer 2

x-y=9    y=x-9
2x+y=21
2x+x-9=21
3x=9+21
3x=30
x=10
10-y=9
y=1
x+y= 10+1=11