3^24-1
=(3^12+1)(3^6+1)(3^3+1)(3^3-1)
=(3^12+1)(3^6+1)(3+1)(3^2-3+1)(3-1)(3^2+3+1)
因为3^2-3+1=7
3^2+3+1=13
又因为7和13互质
所以3^24-1能被7*13=91整除
3^24-1
=(3^12+1)(3^6+1)(3^3+1)(3^3-1)
=(3^12+1)(3^6+1)(3+1)(3^2-3+1)(3-1)(3^2+3+1)
因为3^2-3+1=7
3^2+3+1=13
又因为7和13互质
所以3^24-1能被7*13=91整除