x+y=90
_______________
a)
[tex]y=\frac{1}{8}*x[/tex]
[tex]=> x+y= 90\\=> x+\frac{1}{8}*x=90\\=>\frac{8x+x}{8}=90\\=> \frac{9x}{8}=90\\\\=> 9x=720\\=> x=80[/tex]
80+y=90
=> y=10
b)
[tex]y=3x[/tex]
[tex]=>x+y=90\\=> x+3x=90\\=> 4x=90\\=> x= 22,5[/tex]
22,5+y=90
=> y= 67,5
c)
[tex]y=\frac{1}{7}*x[/tex]
[tex]=> x+y=90\\=>x+\frac{1}{7}*x=90\\=> \frac{8x}{7} =90\\=> 8x=630\\=> x=78,75[/tex]
78,75+y=90
=> y= 11,25
Sper că ai înțeles!