已知{an}为等差数列,其公差为-2,且a7是a3与a9的等比中项,sn为{an}的前n项和,n∈N﹡,则S10的值为( )
从方框中选择恰当的短语,并用其适当形式填空。
so far thanks to be known as at the same time carry out take measures to