Complex Analysis Multiple Choice Questions on “Continuity”.
1. The function f(x, y)=(frac{x^2(2xy-3x)+9y}{3x-2y}) is which of the following?
a) Discontinuous at origin
b) Discontinuous at (1, 1)
c) Continuous at origin
d) Discontinuous at (2, 2)
Answer: a
Explanation: Origin implies the coordinates (0, 0)
Case 1: (lim_{x to 0 \ y to 0}frac{x^2(2xy-3x)+9y}{3x-2y})
(lim_{y to 0}frac{9y}{-2y})
=(frac{-9}{2})
Case 2: (lim_{y to 0 \ x to 0}frac{x^2(2xy-3x)+9y}{3x-2y})
(lim_{x to 0}frac{-3x^3}{3x})
=0
Since, both the limits are not equal, the function is not continuous at origin.
2. The function f(x, y)=(frac{2xy-3x}{3x-2y}) is continuous at (1, 1).
a) True
b) False
Answer: b
Explanation: Case 1: (lim_{x to 1 \ y to 1}frac{2xy-3x}{3x-2y})
(lim_{y to 1}frac{2y-3}{3-2y})=(frac{-1}{1})=-1
Case 2: (lim_{y to 1 \ x to 1}frac{2xy-3x}{3x-2y})
(lim_{x to 1}frac{2x-3x}{3x-2})=(frac{-1}{1})=-1
Case 3: (lim_{y to mx \ x to 0}frac{2xy-3x}{3x-2y})
(lim_{x to 0}frac{2mx^2-3x}{3x-2mx})=(frac{-3}{3-2m})
Since the cases are not same, f(x, y) is discontinuous at x=1.
3. f(x, y)=(frac{3xy+3y+3x}{5x+5y}) continuous at origin.
a) True
b) False
Answer: b
Explanation: Case 1: (lim_{x to 0 \ y to 0}frac{3xy+3y+3x}{5x+5y})
(lim_{y to 0}frac{3y}{5y})=(frac{3}{5})
Case 2: (lim_{y to 0 \ x to 0}frac{3xy+3y+3x}{5x+5y})
(lim_{x to 0}frac{3x}{5x})=(frac{3}{5})
Case 3: Along a linear path, (lim_{y to mx \ x to 0}frac{3xy+3y+3x}{5x+5y})
(lim_{x to 0}frac{3mx^2+3mx+3x}{5x})=(frac{3(m+1)}{5})
Since, the cases are not equal, f(x, y) is discontinuous at x=0.
4. If f(x)=(frac{x+3}{x^2-5x+6}) and g(x)=(frac{x-3}{x^2-6x+8}), then which of the following is correct?
a) f(x) is continuous at x=2 and g(x) is continuous at x=2
b) f(x) is discontinuous at x=3 and g(x) is discontinuous at x=2
c) f(x) is continuous at x=4 and g(x) is discontinuous at x=4
d) f(x) is discontinuous at x=3 and g(x) is continuous at x=2
Answer: b
Explanation: The denominator of f(x) is (x2-5x=6). When we factorize this, we get,
(x2-5x+6)=x2-2x-3x+6
(x2-5x+6)=(x-2)(x-3)
Since the denominator becomes 0 at x=2 and x=3, f(x) is discontinuous at x=2 and x=3.
The denominator of g(x) is (x2-6x+8). When we factorize this, we get,
(x2-6x+8)=x2-2x-4x+8
(x2-6x+8)=(x-2)(x-4)
Since the denominator becomes 0 at x=2 and x=4, f(x) is discontinuous at x=2 and x=4.
5. If f(x)=(frac{x+3}{x^2-5x})and g(x)=(frac{x-3}{x^2-6x}), then which of the following is correct?
a) f(x) is continuous at x=1 and g(x) is continuous at x=0
b) f(x) is discontinuous at x=6 and g(x) is discontinuous at x=5
c) f(x) is continuous at x=5 and g(x) is discontinuous at x=0
d) f(x) is discontinuous at x=5 and g(x) is continuous at x=5
Answer: b
Explanation: The denominator of f(x) is (x2-5x). When we factorize this, we get,
(x2-5x)=x(x-5)
Since the denominator becomes 0 at x=0 and x=5, f(x) is discontinuous at x=0 and x=5.
The denominator of g(x) is (x2-6x). When we factorize this, we get,
(x2-6x+8)=x(x-6)
Since the denominator becomes 0 at x=0 and x=6, f(x) is discontinuous at x=0 and x=6.
6. The function f(x)=(frac{1}{x^3-3x^2-10x+24})is continuous at which of the following?
a) 1
b) 2
c) 3
d) 4
Answer: a
Explanation: When we substitute x=2, x=3 or x=4 in the function, we get the denominator as 0. There by, making the function discontinuous at these points.
At x = 1, we get f(1)=(frac{1}{12}) which is continuous.
7. The function f(x)=(frac{e^{-x}}{x(x-5)(x+2)}) is continuous at which of the following?
a) 0
b) 2
c) 5
d) The function is continuous at all points
Answer: b
Explanation: When we substitute x=0, x=5 or x=-2 in the f(x), we get the denominator as 0. This is because these are the points of discontinuity.
At x=2, we get f(2)=(frac{-1}{24}) which is continuous.
8. Which of the following is true about f(x)=tanx?
a) Continuous at x=(frac{pi}{2})
b) Continuous at x=(-frac{3pi}{2})
c) Continuous at x=0
d) Continuous at x=(frac{3pi}{2})
Answer: c
Explanation: tanx=(frac{sinx}{cosx})
cosx=0 when x=(frac{pm (n+1)pi}{2}) which are points of discontinuity for the function, f(x)=tanx.
9. Which of the following is true about f(x)=sinx+cosx?
a) Continuous everywhere
b) Continuous at x=(frac{pm npi}{2}), where n is any integer
c) Continuous at x=(frac{pm (n+1)pi}{2}), where n is any integer
d) Continuous at x=0
Answer: a
Explanation: The values of sinx and cosx never become infinity or non-determinable for any value of x. Therefore, the function f(x)=sinx+cosx is continuous at all points of x.
10. Which of the following is false if both f(x) and g(x) are continuous?
a) f(x)+g(x) is continuous
b) f(x)-g(x)is continuous
c) f(x)*g(x)is continuous
d) (frac{f(x)}{g(x)}) is continuous
Answer: d
Explanation: For (frac{f(x)}{g(x)}) to be continuous, the denominator needs to be greater than 0. If not, the function becomes infinity and cannot be determined, thereby becoming discontinuous.
11. Which of the following is discontinuous at x=9?
a) (frac{x^2-81}{x-9})
b) (frac{x^3-20x^2+117x-162}{x-9})
c) (frac{x^2-11x+18}{x(x-9)})
d) (frac{x^2-13x-90}{x-9})
Answer: d
Explanation: (frac{x^2-81}{x-9})=(frac{(x-9)(x+9)}{x-9})=(x-9) which is continuous at x = 9.
(frac{x^3-20x^2+117x-162}{x-9})=(frac{(x-9)^2(x-2)}{(x-9)})=(x-9)(x-2) which is continuous at x = 9.
(frac{x^2-11x+18}{x(x-9)})=(frac{(x-2)(x-9)}{x(x-9)})=(frac{(x-2)}{x}) which is continuous at x = 9.
(frac{x^2-13x-90}{x-9})=(frac{(x-18)(x+5)}{x-9}) which is discontinuous at x = 9, since the denominator becomes 0.
12. Which of the following functions are continuous at x=11?
a) (frac{1}{x^2+x-132})
b) (frac{1}{x^3-7x^2-40x-44})
c) (frac{1}{x^2-2x-143})
d) (frac{x-11}{x^3-23x^2+143x-121})
Answer: c
Explanation: (frac{1}{x^2+x-132})=(frac{1}{(x-11)(x+12)}) is discontinuous at x=11 since the denominator becomes 0.
(frac{1}{x^3-7x^2-40x-44})=(frac{1}{(x-11)(x+2)^2})is discontinuous at x=11 since the denominator becomes 0.
(frac{x-11}{x^3-23x^2+143x-121})=(frac{(x-11)}{(x-11)^2(x-1)})=(frac{1}{(x-11)(x-1)})is discontinuous at x=11 since the denominator becomes 0.
(frac{1}{x^2-2x-143})is continuous at x=11.
Global Education & Learning Series – Complex Analysis.
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