∵an=n2（cos2nπ3-sin2nπ3）=n2cos2nπ3∴S30＝12•cos2π3+22cos4π3+32cos2π+…+302cos20π=−12×1−12×22+32−12×42−12×52+62+…−12×282−12×292+302=−12[1+22-2×32）+（42+52-62×2）+…+（282+292...