大家知道,数学中有许多问题有着和谐的对称美,如等差数列{an}的前n项顺序和与逆序和相加,由此巧妙地得到前n项求和公式.解题中如果能善于挖掘与利用这种和谐对称美,往往会有意想不到的收获,配以对偶这种解题技巧就是其中典型的一例. We all know that there are many problems in mathematics with a harmonious symmetry, such as the arithmetic progression {an} the first n terms and reverse order and add, which cleverly get the first n terms of summation formula. Good at mining and using this harmonious symmetry of the United States, there will often be unexpected gains, coupled with the dual problem solving skills is one of the typical examples.