In the domain [ 0 ∘ , 1 0 0 0 ∘ ] , how many solutions are there to sin θ cos θ = 2 1 ?
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sinAcosA = 1/2 only when A = 45 deg so A = {45,225,405,585,765,945} so A has 6 values.
Note that sin θ cos θ = 2 2 sin θ cos θ = 2 sin 2 θ = 2 1 . So we have sin 2 θ = 1 and θ = 2 ( 2 k − 1 ) π for k ∈ Z . The smallest solution that falls in the range [ 0 ∘ , 1 0 0 0 ∘ ] is for k = 1 and the largest is for k = 6 . So there are 6 solutions.
take 2 to left hand side... this makes \sin \2theta=1 hence we get \theta=pi/4 for solutions till 1000 degrees add pi to pi/4..
Doing some Algebra manipulation we get:
2 sin θ cos θ = 1 → sin 2 θ = 1
We know that if θ = 9 0 , 2 7 0 then the equation is true.
So the general solution is 4 5 + 1 8 0 n will be the general solution.
0 ≤ 4 5 + 1 8 0 n < 1 0 0 0 Solving for nonegative integers we get:
0 ≤ n ≤ 5
Thus, 6 possibilities.
sin
θ
cos
θ
=
2
1
simplifies into:
2
sin
θ
cos
θ
=
1
Using the trig identity:
2
sin
θ
cos
θ
=
sin
2
θ
=
1
sin
2
θ
=
1
when
2
θ
=
9
0
d
e
g
r
e
e
s
+
3
6
0
d
e
g
r
e
e
s
This happens when
θ
=
4
5
d
e
g
r
e
e
s
+
1
8
0
d
e
g
r
e
e
s
The solutions are the numbers: 45, 225, 405, 585, 765, and 945 degrees.
This means there are
6
s
o
l
u
t
i
o
n
s
.
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The expression reminds us of sin 2 θ which is 2 × sin θ cos θ . We multiply both sides of the expression by two to come up with the neat expression of sin 2 θ = 1 . Sine is 1 when the angle is 90 or co terminal to 90. The range of 2 θ is 0 to 2,000. Thus we have 90, 450, 810, 1170, 1530, and 1890 or 6 angles.