cos²25°-cos²35°+cos²45°-cos²55°+cos²65°

`#huy`

`cos^(2)25^o - cos^(2)35^o + cos^(2)45^o - cos^(2)55^o + cos^(2)65^o`

`=(cos^(2)25^o + cos^(2)65^o)-(cos^(2)35^o + cos^(2)55^o)+cos^(2)45^o`

`=(cos^(2)25^o + sin^(2)25^o)-(cos^(2)35^o + sin^(2)35^o)+((\sqrt{2})/2)^2`

`=1-1+1/2`

`=1/2`

`#tnvt`

`cos^2 25^0-cos^2 35^0+cos^2 45^0-cos^2 55^0+cos^2 65^0`

`=cos^2 25^0-cos^2 35^0+cos^2 45^0- sin^2 (90^0-55^0)+sin^2 (90^0-65^0)`

`=cos^2 25^0-cos^2 35^0 +cos^2 45^0 -sin^2 35^0+sin^2 25^0`

`=(cos^2 25^0+sin^2 25^0)-(cos^2 35^0+sin^2 35^0)+(cos45^0)^2`

`=1-1+(\frac{\sqrt{2}}{2})^2`

`=\frac{2}{4}`

`=1/2`

Ta áp dụng công thức : `sin^2a+cos^2a=1`