GATE EC 2008


Question 1
All the four entries of the 2x2 matrix P=\begin{bmatrix} P_{11} &P_{12} \\ P_{21} & P_{22} \end{bmatrix} are nonzero, and one of its eigenvalue is zero. Which of the following statements is true?
A
P_{11}P_{22}-P_{12}P_{21}=1
B
P_{11}P_{22}-P_{12}P_{21}=-1
C
P_{11}P_{22}-P_{12}P_{21}=0
D
P_{11}P_{22}+P_{12}P_{21}=0
Engineering Mathematics   Linear Algebra
Question 1 Explanation: 
Eigenvalues are the roots of the determinant formed by matrix [sI-P]
\begin{array}{l} [sI-P]=\left[\begin{array}{cc}s-P_{11} & -P_{12} \\-P_{21} & s-P_{22}\end{array}\right] \\\; [s I - P]=0 \\ \Rightarrow \quad \left(s-P_{11}\right)\left(s-P_{22}\right)-P_{12} P_{21}=0\\ \Rightarrow \quad s^{2}-\left(P_{11}+P_{22}\right) s+P_{11} P_{22}-P_{12} P_{21}=0 \end{array}
Since, one of the its eigenvalues is zero, therefore,
putting s=0
P_{11} P_{22}-P_{12} P_{21}=0
which is the desired condition.
Question 2
The system of linear equations
4x + 2y = 7
2x + y = 6
has
A
a unique solution
B
no solution
C
an infinite number of solutions
D
exactly two distinct solutions
Engineering Mathematics   Linear Algebra
Question 2 Explanation: 
The system can be written in matrix from as
\left[\begin{array}{ll}4 & 2 \\ 2 & 1\end{array}\right]\left[\begin{array}{l}x \\ y\end{array}\right]=\left[\begin{array}{l}7 \\6 \end{array}\right]
The Augmentod matrix [AIB] is given by
\left[\begin{array}{ll|l}4 & 2 & 7 \\ 2 & 1 & 6\end{array}\right]
Performing Gauss elimination on this [A \mid B] as follows:
\left[\begin{array}{ll|l}4 & 2 & 7 \\2 & 1 & 6 \end{array}\right] \frac{R_{2}-\frac{2}{4} R_{1}}{=R_{2}-\frac{1}{2} R_{1}}\left[\begin{array}{ll|l}4 & 2 & 7 \\0 & 0 & 5 / 2 \end{array}\right]
Now Rank [A \mid B]=2
(The number of non-zero rows in [ A|B]
Rank [A]=1
(The number of non-zero rows in [A])
since, Rank [A \mid B] \neq \text{Rank}[A].
The system has no solution.


Question 3
The equation sin(z) = 10 has
A
no real or complex solution
B
exactly two distinct complex solutions
C
a unique solution
D
an infinite number of complex solutions
Engineering Mathematics   Complex Analysis
Question 3 Explanation: 
sinz can have value between -1 to +1. Thus no solution.
Question 4
For real values of x, the minimum value of the function f(x)=exp(x)+exp(-x) is
A
2
B
1
C
0.5
D
0
Engineering Mathematics   Calculus
Question 4 Explanation: 
f(x)=e^{x}+e^{-x}=e^{x}+\frac{1}{e^{x}}
Arithmetic mean of e^{x} and \frac{1}{e^{x}} is
\mathrm{A.M},=\frac{e^{x}+\frac{1}{e^{x}}}{2}
Geometric mean of e^{x} and \frac{1}{e^{x}} is
G.M. =e^{x} \cdot \frac{1}{e^{x}}=1
It is known that A.M. \geq G.M
\frac{\left(e^{x}+\frac{1}{e^{x}}\right)}{2} \geq 1
e^{x}+\frac{1}{e^{x}} \geq 2
Therefore, \left(e^{x}+e^{-x}\right)_{\min }=2
Question 5
Which of the following functions would have only odd powers of x in its Taylor series expansion about the point x = 0 ?
A
sin(x^{3})
B
sin(x^{2})
C
cos(x^{3})
D
cos(x^{2})
Engineering Mathematics   Calculus
Question 5 Explanation: 
\begin{array}{l} \sin x=x-\frac{x^{3}}{3}+\frac{x^{5}}{5}-\frac{x^{7}}{7}+\ldots . \\ \cos x=1-\frac{x^{2}}{2}+\frac{x^{4}}{4}-\frac{x^{6}}{6}+\ldots . \end{array}




There are 5 questions to complete.