`tan B=2=cot\alpha => tan\alpha=1/(cot\alpha)=1/2`
`C=sin^2\alpha+2sin\alpha.cos\alpha-3cos^2\alpha`
Có: `1/(cos^2\alpha)=1+tan^2\alpha => cos^2\alpha=1/(1+tan^2\alpha)` (`cos\alpha\ne0`)
`C=cos^2\alpha((sin^2\alpha)/(cos^2\alpha)+(2sin\alpha.cos\alpha)/(cos^2\alpha)-3)`
`C=1/(1+tan^2\alpha).(tan^2\alpha+2tan\alpha-3)`
`C=1/((1/2)^2+1). ((1/2)^2+2. 1/2-3)`
`C=4/5. (1/4+1-3)`
`C=4/5. (-7/4)=-7/5`
Vậy `C=-7/5`
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`D=(sin^2\alpha-sin\alpha.cos\alpha-cos^2\alpha)/(2sin\alpha.cos\alpha)`
`D=(sin^2\alpha)/(2sin\alpha.cos\alpha)-1/2-(cos^2\alpha)/(2sin\alpha.cos\alpha)`
`D=1/2.tan\alpha-1/2-1/2. cot\alpha`
`D=1/2. 1/2-1/2-1/2. 2`
`D=1/2-1/2-1`
`D=-1`
Vậy `D=-1`