Question

一条高一数学题

Answer 1

a属于第二象限
则cos2a有负有正
sina为正
sina=cos2a
sina=1-2sina^2
设sina=x
则2X^2+X-1=0
解X，X的值便是sina的值
2x^2
-
x
-
1
=
0
2x^2
+
x
-
2x
-
1
=
0
(2x^2
+
x)
-
(2x
+
1)
=
0
x(2x
+
1)
-
1(2x
+
1)
=
0
(2x
+
1)(x
-
1)
=
0
2x
+
1
=
0
2x
=
-1
x
=
-1/2
(-0.5)
x
-
1
=
0
x
=
1
有因sina为正，所以sina=1
tan90不存在
所以我认为是不是a不在第二象限
a应该为150°
tan150°=-根号3/3