sqrt:算数平方根
sqrt(81*82*83*84+1)
=sqrt[(82*83)*(81*84)+1]
=sqrt[6806*6804+1]
=sqrt[(6804+2)*6804+1]
=sqrt[6804*6804+2*6804*1+1]
=sqrt[(6804+1)*(6804+1)]
=6805