$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

`J = tan10^o*tan20^o ...tan80^o`

`=(tan10^o*tan80^o)(tan20^o*tan70^o)(tan30^o*tan60^o)(tan40^o*tan50^o)`

`=(tan10^o*cot10^o)(tan20^o*cot20^o)(tan30^o*cot30^o)(tan40^o*cot40^o)`

`=1`

`J=tan10^0.tan20^0. ... .tan 80^0`

`=(tan10^0.tan80^0) . (tan20^0.tan70^0). (tan30^0 . tan60^0) . (tan 40^0 . tan 50^0)`

`=(tan10^0.cot10^0) . (tan20^0.cot20^0). (tan30^0.cot30^0). (tan40^0.cot40^0)`

`=1`

Công thức `tana^0=cot(90^0-a^0)`

`tana . cot a =1`