How to solve this inequality correctly?

Question

Find the value range of real number a, so that for any real number x and $\theta \in\left[0, \frac{\pi}{2}\right]$, make $(x+3+2 \sin \theta \cos \theta)^{2}+(x+a \sin \theta+a \cos \theta)^{2} \geqslant \frac{1}{8}$ constant.

ForAll[{θ, x}, 
 0 <= θ <= Pi/
  2, (x + 3 + 2 Sin[θ] Cos[θ])^2 + (x + 
     a*Sin[θ] + a*Cos[θ])^2 >= 1/8]
Resolve[%, Reals]

But the above code has been running, can not get the correct result. How to solve this problem correctly?

Reference answer:

Answer 1

We set $y=\cos\theta$ and $z=\sin\theta$,then according to the conditions, $y^2+z^2==1$ and $y\geq 0, z\geq 0$.

And we consider the inverse problem,that is Exists $x,y,z$ such that $$(x + 3 + 2 zy)^2 + (x + az + ay)^2 \leq 1/8$$

Method one

Maximize[{a, (x + 3 + 2 z*y)^2 + (x + a*z + a*y)^2 <= 1/8, y >= 0, 
  z >= 0, y^2 + z^2 == 1}, {x, y, z, a}]

7/2, {x -> -(13/4), y -> 0, z -> 1, a -> 7/2}}

Minimize[{a, (x + 3 + 2 z*y)^2 + (x + a*z + a*y)^2 <= 1/8, y >= 0, 
  z >= 0, y^2 + z^2 == 1}, {x, y, z, a}]

{Sqrt[6], {..., a -> Sqrt[6]}}

Method two

Resolve[Exists[{x, y, 
   z}, {(x + 3 + 2 z*y)^2 + (x + a*z + a*y)^2 <1/8, y >= 0, z >= 0, 
   y^2 + z^2 == 1}], Reals]

Sqrt[6] < a < 7/2

Answer 2

According to ForAll documentation, when you use multiple variables, you can't specify a satisfying condition, your ForAll code has one extra argument:

Resolve[ForAll[
  x, (x + 3 + 2 Sin[\[Theta]] Cos[\[Theta]])^2 + (x + 
        a*Sin[\[Theta]] + a*Cos[\[Theta]])^2 >= 1/8 && 
   0 <= \[Theta] <= Pi/2], Reals]

Out:

Answer 3

Executing an equivalent form of your code

Reduce[ForAll[{x, \[Theta]}, (x + 3 + 
    2 Sin[\[Theta]] Cos[\[Theta]])^2 + (x + a*Sin[\[Theta]] + 
    a*Cos[\[Theta]])^2 >= 1/8 && \[Theta] >= 0 && \[Theta] <= 
Pi/2], a, Reals]

, one obtains

...Reduce:This system cannot be solved with the methods available to Reduce

However, this can be done in two steps. First,

Reduce[ForAll[x, (x + 3 + 2 Sin[\[Theta]] Cos[\[Theta]])^2 + (x + 
     a*Sin[\[Theta]] + a*Cos[\[Theta]])^2 >= 1/8 && \[Theta] >= 
 0 && \[Theta] <= Pi/2], a, Reals] // Simplify

(\[Theta] >= 0 && 2 \[Theta] < \[Pi] && (a <= (5 + 4 Cos[\[Theta]] Sin[\[Theta]])/( 2 (Cos[\[Theta]] + Sin[\[Theta]])) || a >= (7 + 4 Cos[\[Theta]] Sin[\[Theta]])/( 2 (Cos[\[Theta]] + Sin[\[Theta]])))) || (2 \[Theta] == \[Pi] && (2 a <= 5 || 2 a >= 7))

produces the required bounds for concrete values of \[Theta].

Second, one finds the bounds valid for all $\theta \in [0,\pi/2]$ by

Minimize[{(5 + 4 Cos[\[Theta]] Sin[\[Theta]])/(2 (Cos[\[Theta]] + Sin[\[Theta]])), 
[Theta] >= 0 && \[Theta] <= Pi/2}, \[Theta]]

{(5 + 4 Cos[2 ArcTan[ Root[1 - 8 # + 2 #^2 + 8 #^3 + #^4& , 3, 0]]] Sin[2 ArcTan[ Root[1 - 8 # + 2 #^2 + 8 #^3 + #^4& , 3, 0]]])/(2 (Cos[2 ArcTan[ Root[1 - 8 # + 2 #^2 + 8 #^3 + #^4& , 3, 0]]] + Sin[2 ArcTan[ Root[1 - 8 # + 2 #^2 + 8 #^3 + #^4& , 3, 0]]])), {\[Theta] -> 2 ArcTan[ Root[1 - 8 # + 2 #^2 + 8 #^3 + #^4& , 3, 0]]}}

which produces the upper bouds for a in terms of quadric and this can be expressed through radicals. To obtain numbers,

N[%]

{2.44949, {\[Theta] -> 0.261799}}

The lower bound for a is simpler.

Maximize[{(7 + 4 Cos[\[Theta]] Sin[\[Theta]])/(2 (Cos[\[Theta]] + Sin[\[Theta]])), \
[Theta] >= 0 && \[Theta] <= Pi/2}, \[Theta]]

{7/2, {\[Theta] -> 0}}

Therefore, since $2.44949 <5/2$, there is no bounds for a which are valid for all the $\theta \in [0,\pi/2]$.