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9) B

10) 5x + 7y = 12 |×3
3x - y = 2 |×5
maka
15x + 21y = 36
15x - 5y = 10
---------------------(-)
26y = 26
y = 1
subtitusi
3x - y = 2
3x - 1 = 2
3x = 2 + 1
3x = 3
x = 1
nilai 8x + 6y
= 8(1) + 6(1)
= 8 + 6
= 14 ( D )

11) x + y = 4
x - y = 5
x² - y² = ( x + y )( x - y)
= (4 )( 5)
= 20 ( C )