6.
a) x:y=2:3 ⇔ x=2/3 .y
Lại có: x-y=-2⇔ 2y/3-y=-2⇔-y/3=-2⇔y=6
⇒x=2/3 .6 =4
Vậy x=4; y=6
b)x=3/4 .y ⇒xy=3/4 . y2=48
⇔y2=64 ⇔[y=8⇒x=6y=−8⇒x=−6
7.
a) x=2/5 .y ; z= 4/3 . y
⇒x+y+z= 2/5 .y +y+ 4/3 . y=-82
⇔41/15.y=82
⇔y=30
⇒x=12;z=40
b) Ta có:
(x−1)2≥0
(y−1/2)2≥0
(z−2020)2≥0
⇒(x−1)2+(y−1/2)2+(z−2020)2≥0
Vậy pt⇔ (x−1)2=0;(y−1/2)2=0;(z−2020)2=0
⇔x=1; y=1/2; z=2020
