` \tan20^0 =1/\cot20^0`
`=> \tan20^0× \cot20^0 = 1`
`\sin∝ = \cos(90^0-∝)`
`\cos∝ = \sin(90^0-∝)`
`\sin^2 ∝+ \cos^2 ∝= 1`
`=>VT = 1 / {cos10^0/sin10^0 + cos80^0/sin80^0}`
`= {sin10^0×sin80^0}/{cos10^0×sin80^0+sin10^0×cos80^0}`
`= {sin10^0×sin80^0} / {sin(90^0-10^0)×sin80^0+cos(90^0-10^0)×cos80^0}`
`= {sin10^0×sin80^0} / {(sin80^0)^2+(cos80^0)^2}`
`= sin10^0×sin80^0` `= VP ( đpcm)`
