設 x, y 為實數, x+2y= 3, x^3+8y^3 =9, 求 x^2+ 4y^2=?
回答 (2)
2015-09-16 2:50 pm
Ans=5 ,
Let x be 1, y be 1 ,
x + 2y = 1+2 =3 ,
x^3 + 8y^3 =1+8*1= 9
Let x be 1, y be 1 ,
x + 2y = 1+2 =3 ,
x^3 + 8y^3 =1+8*1= 9
2015-09-16 3:01 pm
Sol
x+2y=3
x^3+8y^3=(x+2y)(x^2－2xy+4y^2)
x^2－2xy+4y^2=3
x^2+4xy+4y^2=9
xy=1
x^2+4y^2=9－4=5
x+2y=3
x^3+8y^3=(x+2y)(x^2－2xy+4y^2)
x^2－2xy+4y^2=3
x^2+4xy+4y^2=9
xy=1
x^2+4y^2=9－4=5
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https://hk.answers.yahoo.com/question/index?qid=20150916064413AACkvS8