Question

y²+3x=7,y³+x=3,求y等于几.

Answer 1

y²+3x=7 x=(7-y²)/3

y³+x=3 x=3-y³

(7-y²)/3=3-y³

3y³-y²-2=0

3y³-3y²+2y²-2=0

3y²(y-1)+2(y+1)(y-1)=0

(y-1)(3y²+2y+2)=0

3y²+2y+2=3(y+ 1/3)²+5/3恒>0,因此只有y-1=0 y=1

x=3-y³=3-1=2

y=1