1) $VT = tan\alpha +\frac{1}{cos\alpha } =\frac{sin\alpha }{cos\alpha }+\frac{1}{cos\alpha }=\frac{1+sin\alpha }{cos\alpha }=\frac{(1+sin\alpha ).(1-sin\alpha )}{cos\alpha .(1-sin\alpha )}=\frac{1-sin^{2}\alpha }{cos\alpha .(1-sin\alpha )}= \frac{1}{1-sin\alpha }=VP (đpcm).$
1) $VT = tan\alpha +\frac{1}{cos\alpha } =\frac{sin\alpha }{cos\alpha }+\frac{1}{cos\alpha }=\frac{1+sin\alpha }{cos\alpha }=\frac{(1+sin\alpha ).(1-sin\alpha )}{cos\alpha .(1-sin\alpha )}=\frac{1-sin^{2}\alpha }{cos\alpha .(1-sin\alpha )}= \frac{cos\alpha }{1-sin\alpha }=VP (đpcm).$