You may also be having problems with the synchro rings that allow
the gears to mesh without the grinding. 1st-2nd gear share a
synchro and 3rd-4th gear share a synchro too. It sounds like the
3rd-4th gear synchro may be worn.
Sounds like the synchros are failing. Usually a time consuming fix for a professional. Estimate about $1000.00 for a repair if that's the problem. Grinding in all but 4th and 5th is an indication of a bad input shaft bearing
If "a" is positive, it will have two fourth roots, one will be positive and one will be negative it will have one fifth root, which will be positive. If "a" is negative, it will have one fourth root, which will be negative. it will have one fifth root, which will be negative.