√(81*82*83*84+1)
=√[(82*83)*(81*84)+1]
=√[6806*6804+1]
=√[(6804+2)*6804+1]
=√[6804*6804+2*6804*1+1]
=√[(6804+1)*(6804+1)]
=6805
√(81*82*83*84+1)
=√[(82*83)*(81*84)+1]
=√[6806*6804+1]
=√[(6804+2)*6804+1]
=√[6804*6804+2*6804*1+1]
=√[(6804+1)*(6804+1)]
=6805