1.已知a+b+c=1 求证:1/a+1/b+1/c大于等于9 (a b c 属于R正)2.已知a+b+c+d=1 求证 a^2+b^2+c^2+d^2大于等于1/4(a b c d属于R正)3.设x1 x2......xn是正数求证:(x1+x2+.....xn)(1/x1+1/x2+......1/xn)大于等于n^2谢谢~~求过程~!