证明：对于任意实数x、y，都有 x^4+y^4≥(1/2)xy(x+y)^ (x^4+y^4)-(1/2)xy(x+y)^ =(1/2)[2x^4+2y^4-xy(x^+2xy+y^)] =(1/2)[2x^4+2y^4-x^3*y-2x^y^-xy^3] =(1/2)[(x^4-x^3*y)-(xy^3-y^4)+(x^4-2x^y^+y^4)] =(1/2)[(x-y)(x^3-y^3)+(x^-y^)^] =(1/2)(x-y)^*[(x^+xy+y^)+(x+y)^] =(1/2)(x-y)^*[(x+y/2)^+3y^/4+(x+y)^]≥0 ---> x^4+y^4≥(1/2)xy(x+y)^
答：要证明x^4+x^4≥1/2xy(x+y)^2 只需要证2x^4+2x^4≥xy(x+y)^2 =xy(x^2+2xy+y^2) 2x^4+2y^4≥xy(x+...详情>>
答：C 因为still表示“仍然”，说话的人原以为某事已经结束、某人已经离开等，而结果却相反。still 在应该放在句字中要修饰的动词的前面或后面。 注意still...详情>>
问：It’s time to hanve an English exam。 The ...
答：be busy doing sth 后面的prepare for sth 又是个词组，表示为……作准备详情>>
