Values for $a$ and $b$ in $y=\cos(x)+a\cos(bx)$ such that every real value for $x$ has either a positive or $0$ value for $y$ If there is a function in the form $y=\cos(x)+a\cos(bx)$ do there exists real number values for $a$ and $b$ such that for every real number value for $x$ there is either a positive number value for $y$ or a $0$ value for $y$?
 A: Let $f(x) = \cos(x)+a\cos(bx)$, and suppose $f(x) \ge 0$ for all $x \in \mathbb{R}$.

First suppose $b=0$. Then $f(x) = \cos(x) + a,$ which has range $[a-1,\infty).$

Thus, for $b=0$, we get the solution pairs $(a,b) = (t,0),\,$ for any $t \ge 1.$

Next, suppose $b \ne 0$.

If $|a| > 1$, then when $a\cos(bx)$ realizes a value of $-a$, $f(x)$ will be negative, contradiction.

Similarly, if $|a| < 1$, then when $\cos(x)$ realizes a value of $-1$, $f(x)$ will be negative, contradiction.

It follows that $|a| = 1,$ hence either
\begin{align*}
f(x) &= \cos(x) + \cos(bx)\\[4pt]
&\,\text{or}\\[4pt]
f(x) &= \cos(x) - \cos(bx)\\[4pt]
\end{align*}
First suppose $f(x) = \cos(x) + \cos(bx)$. 

Then when one of the summands realizes a value of $-1$, the other must realize a value of $+1$ to compensate. But that means each of the summands has a period which is a multiple of the other. It follows that periods are equal, hence $b = \pm 1$. But $\cos\,$ is an even function hence, $b = \pm 1$ implies $\cos(bx) = \cos(x)$. But then $f(x) = 2\cos(x)$, contradiction, since then $f(\pi) < 0$.

Next suppose $f(x) = \cos(x) - \cos(bx)$. 

If $x_0$ is such that $\cos(x_0) = -1$, then $\cos(bx_0)$ must also be $-1$, else $f(x_0) < 0$.

It follows that the period of $\cos(bx)$ divides the period of $\cos(x)$.

Similarly, if $x_0$ is such that $\cos(bx_0) = 1$, $\cos(x_0)$ must also be $1$, else $f(x_0) < 0$.

It follows that the period of $\cos(x)$ divides the period of $\cos(bx)$.

Therefore the periods must be equal, hence $b = \pm 1$. But then, since $\cos$ is an even function, $b = \pm 1$ implies $\cos(bx) = \cos(x)$, hence $f(x)$ is identically $0$.

This gives the solution pairs $(a,b) = (-1,\pm 1).$

To summarize, we have the solution pairs
\begin{align*}
(a,b) &= (-1,1)\\[4pt]
(a,b) &= (-1,-1)\\[4pt]
(a,b) &= (t,0),\,\text{for any $t \ge 1$}\\[4pt]
\end{align*}
and those are the only solutions.
A: Assuming $b \not =  0$.
If you allow $y = 0$, then yes. Otherwise, no:
$\int cos(x) + a cos(bx) dx = sin (x) + (a/b)sin (bx)$, integrate again to get $- cos(x) - (a / b^2) cos(bx)$. Keep repeating this to get the integral $sin(x) + a / b^n sin(bx).$ If $b \not = 1$, then either $a / b^n$ is very very small, in which case $sin(x) $ dominates, or it is very big, in which case $a / b^n sin(bx)$ dominates. In either of these cases,the integral will have regions on which it is positive or negative, meaning the integrand had to have regions in which it was positive or negative, and inducting back we see that $cos(x) + a cos(bx)$ had to be positive in some places, negative in others. 
In the case $b = 1$, we have just $(a + 1) cos(x)$...
A: Just set $b=0$ and $a=10$, then
$$
y = \cos (x) + a \cos (bx) = \cos (x) + 10 \cos (0) =\cos (x)+10\geq -1 +10 =9>0
$$
for all real $x $.
A: Let $f(x)=\cos(x)+a\cos(bx)$. We will show:

If $f$ is non negative then $f\equiv 0$ (*)

First, assume that $b$ is rational, say $b=p/q$ (in lowest terms). Then
$$\int_0^{q\pi}f(x)dx=\int_0^{q\pi}[\cos(x)+a\cos(bx)]dx=0$$
which is implies (*).
If $b$ is irrational, let $\epsilon>0$. See Dirichlet's approximation theorem, if needed. There are integer numbers $p,q$ with $q>\max\{1/\sqrt\epsilon,|a/b|\}$, such that
$$\left|b-\frac pq\right|<\frac1{q^2}$$
Then, using that $|\sin'|\le 1$ and by the mean value theorem,
$$\begin{align}
\left|\int_0^{q\pi}f(x)dx\right|&=\left|\int_0^{q\pi}[\cos(x)+a\cos(bx)]dx\right|\\
&=\left|\frac ab\sin(bq\pi)\right|\\
&=\left|\frac ab[\sin(bq\pi)-\sin(p\pi)]\right|\\
&\le\left|\frac ab(bq-p)\right|\pi<\frac \pi q<\pi\sqrt\epsilon
\end{align}$$
So, if $f$ were non negative, then
$$\sup f<\frac{\pi\sqrt\epsilon}{q\pi}<\epsilon$$
and $f\equiv 0$.
A: We assume $b\ne0$ and $|b|\ne 1$ for (hopefully) obvious reasons.
If there is a solution,
$$I(t):=\int_0^t(\cos x+a\cos bx)\,dx=\sin t+\frac ab\sin bt$$ must be increasing, and as it is bounded, must converge. This is not possible.
