Câu 4:
Có \(36^0+54^0=90^0\Rightarrow sin36^0=cos54^0\)và \(sin54^0=cos36^0\)
\(sin36^0-cos54^0+3tan36^0.tan54^0-sin^236^0-cos^254^0\)
\(=0+3\dfrac{sin36^0}{cos36^0}.\dfrac{sin54^0}{cos54^0}-\left(sin^236^0+cos^254^0\right)\)
\(=3.\dfrac{cos54^0}{cos36^0}.\dfrac{cos36^0}{cos54^0}-2sin^236^0\)\(=3-2sin^236^0\)\(\approx2,3\)
Câu 5:
Vì \(sin\alpha=2>1\)\(\Rightarrow\alpha\in\varnothing\)
Không tính được \(cos\alpha\)
Câu 6:
\(cot\alpha=\dfrac{1}{tan\alpha}=\dfrac{1}{2}\)
Câu 7:
\(\dfrac{sin^2\alpha-cos^2\alpha}{1-2cos^2\alpha}=\dfrac{\left(sin^2\alpha+cos^2\alpha\right)-2cos^2\alpha}{1-2cos^2\alpha}=\dfrac{1-2cos^2\alpha}{1-2cos^2\alpha}=1\) ( dpcm)
