在等比数列中,,且前项和,则此数列的项数等于( ) A. B. C. D. 答案:B 【解析】 由等比数列的性质得出,结合,得出和的值,并设等比数列的公比为,由,求出的值,然后利用等比数列的通项公式可求出的值. 【详解】设等比数列的公比为,由等比数列的性质可得:, 又,和是方程的两根,解方程得或. 若等比数列递增,则, , 解得,解得; 若等比数列递减,则, ,The calculations, had they been done by hand,
______all practical value by the time they were finished.
A. had lost B. would lose C. should
have lost D. would have lost