tính các giá trị biểu thức sau thức sau:
D= cot 20.cot45.cot70/tan20.tan45.tan70
B= tan 1.tan 2.tan 3 ........ tan 87. tan 88.tan 89
$\cot \alpha = \tan (90^o-\alpha)$
$D=\dfrac{\cot 20.\cot 45.\cot 70}{\tan 20.\tan 45.\tan 70}=\dfrac{\tan 70.\tan 45.\tan 20}{\tan 20.\tan 45.\tan 70}=1$
$B=\tan 1.\tan 2.\tan 3.....\tan 87.\tan 88.\tan 89\\=\tan 1.\tan 2.\tan 3.....\tan 44.\tan 45.\cot 44.\cot 43.....\cot 3.\cot 2.\cot 1\\=(\tan 1.\cot 1).(\tan 2.\cot 2).....(\tan 44.\cot 44).\tan 45\\=1.1.....1.1\\=1$