Bài 7. Chứng minh các hệ thức sau:
a) \( = \)
b) \({{\sin a + \sin 3a + \sin 5a} \over {\cos a + \cos 3a + \cos 5a}} = \tan 3a\)
c) \({{{{\sin }^4}a - {{\cos }^4}a + {{\cos }^2}a} \over {2(1 - \cos a)}} = {\cos ^2}{a \over 2}\)
d) \({{\tan 2x\tan x} \over {\tan 2x - \tan x}} = \sin 2x\)
Trả lời:
a)
\(\eqalign{
& = {{{{\cos }^2}a - {{\sin }^2}a} \over {{{\cos }^2}a + {{\sin }^2}a + 2\sin a\cos a}} \cr
& = {{\cos a - \sin a} \over {\cos a + \sin a}} = } \over {1 + {{\sin a} \over {\cos a}}}} \cr
& = \cr} \)
b)
\(\eqalign{
& {{\sin a + \sin 3a + \sin 5a} \over {\cos a + \cos 3a + \cos 5a}} \cr
& = {{2\sin = {{\sin 3a(1 + 2\cos 2a)} \over {\cos 3a(1 + 2\cos 2a)}} \cr
& = \tan 3a \cr} \)
c)
\(\eqalign{
& {{{{\sin }^4}a - {{\cos }^4}a + {{\cos }^2}a} \over {2(1 - \cos a)}} = {{({{\sin }^2}a + {{\cos }^2}a)({{\sin }^2}a - {{\cos }^2}a) + {{\cos }^2}a} \over {2(1 - \cos a)}} \cr
& = {{{{\sin }^2}a - {{\cos }^2}a + {{\cos }^2}a} \over {4{{\sin }^2}{a \over 2}}} = {{4{{\sin }^2}{a \over 2}{{\cos }^2}{a \over 2}} \over {4{{\sin }^2}{a \over 2}}} \cr
& = {\cos ^2}{a \over 2} \cr} \)
d) 
\(\eqalign{
& {{\tan 2x\tan x} \over {\tan 2x - \tan x}} \cr
& = {{{{2\tan x} \over {1 - {{\tan }^2}x}}.\tan x} \over {{{2\tan x} \over {1 - {{\tan }^2}x}} - \tan x}} = {{2\tan x} \over {{{\tan }^2}x + 1}} \cr
& = \sin 2x \cr} \)