Question

решите системы методом сложения:
1) 2x + y = 20
x - 3y = -1

2)9x + 13y = 31
18x - 5y = 31

Answer 1

2х+у=20
х-3у= -1           ( *-2)

 2x+y=20
-2x+6y=2

7y=22
x=3y-1

y=22/7
x=3 *22/7 - 1

y=3 1/7
x=8 3/7

/
9x+13y=31       ( * -2)
18x -5y=31

-18x - 26y= -62
18x-5y=31

-21y= -31
18x -5y=31

y=31/21
18x=31+5*31/21

y=31/21
18x=38 8/21

y=1 10/21
х=806/21 :18

у=1 10/21
х=2 25/189