Which one is true? (CSIR) Let $a,b,c$ be a positive real number such that $b^2+c^2<a<1$. Let 
$A=\begin{bmatrix} 1&b&c\\ b&a & 0\\ c & 0 & 1\end{bmatrix}$. Then 
(1) all eigen values of $A$ are positive
(2) all eigenvalues of $A$ are negative
(3) all eigenvalues of $A$ are either positive or negative
(4) all eigenvalues of $A$ are nonreal complex number
Since $A$ is symmetric, all eigen values are real. Hence option (4) is not true. 

Using submatrices:
$$\Delta_1=\det\{[1]\}=1>0$$
$$\Delta_2=\det\left\{\left[\begin{array}{cc}1&b\\b&a\end{array}\right]\right\}=a-b^2>a-(b^2+c^2)>0$$
$$\begin{array}{rcl}
\Delta_3&=&\det\left\{\left[\begin{array}{ccc}1&b&c\\b&a&0\\c&0&1\end{array}\right]\right\}\\
&=&(a+0+0)-(ac^2+b^2+0)\\
&=&a-ac^2-b^2>a-c^2-b^2>0
\end{array}$$
Then, all subdeterminants are positive, so $A$ is a positive definite matrix and all its eigenvalues are positive.
