帮忙一下，这有关数学问题
- 提问者网友：情歌越听越心酸
- 2021-07-31 18:36
- 五星知识达人网友：醉吻情书
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an-an-1=5+n-1=4+n
a1=1
an=4(n-1)+(n-1)n/2+1=(n-1)(8+n)/2+1
a13=6*21+1=127
- 1楼网友：酒醒三更
- 2021-07-31 20:12
% MathType!Translator!2!1!LaTeX.tdl!TeX -- LaTeX 2.09 and later! % MathType!MTEF!2!1!+- % feaagaart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaWGbb % Gaeyypa0ZaaSaaaeaacaaIXaaabaGaaGOmaaaacaWGobqdcaaIYaGa % ey4kaSIcdaWcaaqaaiaaiEdaaeaacaaIYaaaaiaad6eaniaaikdacq % GHsislcaaIZaaakeaaniaadgeacaGGOaGaaGymaiaaiodacaGGPaGa % eyypa0JcdaWcaaqaaiaaigdaaeaacaaIYaaaaiaacIcacaaIXaGaaG % 4maiaacMcaniaaikdacqGHRaWkkmaalaaabaGaaG4naaqaaiaaikda % aaGaaiikaiaaigdacaaIZaGaaiyka0GaaGOmaiabgkHiTiaaiodacq % GH9aqpcaaI2aGaaG4naiaaiodaaaaa!5582! \[ \begin{array}{l} A = \frac{1}{2}N2 + \frac{7}{2}N2 - 3 \\ A(13) = \frac{1}{2}(13)2 + \frac{7}{2}(13)2 - 3 = 673 \\ \end{array} \] % MathType!End!2!1!