Решите уравнение 2 cos² x-1=sin x. ​

Based on two different cases:  

x

=

π

6

,

5

π

6

or

3

π

2

Look below for the explanation of these two cases.

Explanation:

Since,  

cos

x

+

sin

2

x

=

1

we have:  

cos

2

x

=

1

−

sin

2

x

So we can replace  

cos

2

x

in the equation  

1

+

sin

x

=

2

cos

2

x

by  

(

1

−

sin

2

x

)

⇒

2

(

1

−

sin

2

x

)

=

sin

x

+

1

or,  

2

−

2

sin

2

x

=

sin

x

+

1

or,  

0

=

2

sin

2

x

+

sin

x

+

1

−

2

or,  

2

sin

2

x

+

sin

x

−

1

=

0

using the quadratic formula:

x

=

−

b

±

√

b

2

−

4

a

c

2

a

for quadratic equation  

a

x

2

+

b

x

+

c

=

0

we have:

sin

x

=

−

1

±

√

1

2

−

4

⋅

2

⋅

(

−

1

)

2

⋅

2

or,  

sin

x

=

−

1

±

√

1

+

8

4

or,  

sin

x

=

−

1

±

√

9

4

or,  

sin

x

=

−

1

±

3

4

or,  

sin

x

=

−

1

+

3

4

,

−

1

−

3

4

or,  

sin

x

=

1

2

,

−

1

Case I:

sin

x

=

1

2

for the condition:  

0

≤

x

≤

2

π

we have:

x

=

π

6

or

5

π

6

to get positive value of  

sin

x

Case II:

sin

x

=

−

1

we have:

x

=

3

π

2

to get negative value of  

sin

x

Answer link

Объяснение: