Tính : $sin^2 45^0-2sin^2 50^0 +3.cos^2 45^0-2.sin^2 40^0 +4.tan55^0.tan35^0$

`sin^2 45^o - 2sin^2 50^o + 3cos^2 45^o - 2sin^2 40^o + 4tan55^o*tan35^o`

`=(sqrt2/2)^2 + 3*(sqrt2/2)^2 - 2sin^2 50^o-2cos^2 50^o + 4tan55^o*cot55^o`

`=2-2(sin^2 50^o + cos^2 50^o)+4`

`=2-2+4`

`=4`