Maths-
General
Easy

Question

In the xy - plane, the graph of 2 x squared minus 6 x plus 2 y squared plus 2 y equals 45 is a circle. What is the radius of the circle?

  1. 5
  2. 6.5
  3. square root of 40
  4. square root of 50

hintHint:

Hint:
  • For the circle, x squared plus y squared plus 2 g x plus 2 f y plus c equals 0, radius will be r equals square root of g squared plus f squared minus c end root.

The correct answer is: 5


    Explanation:
    • We have given the equation of circle 2 x squared minus 6 x plus 2 y squared plus 2 y equals 45
    • We have to find the radius of the given circle.
    • We will first compare the given equation with standard equation x squared plus y squared plus 2 g x plus 2 f y plus c equals 0
    and then apply the formula of radius r equals square root of g squared plus f squared minus c end root
    Step 1 of 2:
    We have given circle 2 x squared minus 6 x plus 2 y squared plus 2 y equals 45.
    Now convert this equation in standard form by making coefficient of x2, y2 as 1
    So, Divide whole equation by 2.
    So, the equation will be
    2 x squared minus 6 x plus 2 y squared plus 2 y equals 45
    fraction numerator 2 x squared minus 6 x plus 2 y squared plus 2 y over denominator 2 end fraction equals 45 over 2
    x squared minus 3 x plus y squared plus y equals 45 over 2
    x squared minus 3 x plus y squared plus y minus 45 over 2 equals 0
    Step 2 of 2:
    On comparing with standard equation, x squared plus y squared plus 2 g x plus 2 f y plus c equals 0.
    So,
    g equals 3 over 2
    f equals 1 half
    c equals fraction numerator negative 45 over denominator 2 end fraction
    So, The radius will be
    r equals square root of g squared plus f squared minus c end root
    r equals square root of open parentheses 3 over 2 close parentheses squared plus open parentheses 1 half close parentheses squared minus open parentheses fraction numerator negative 45 over denominator 2 end fraction close parentheses end root
    r equals square root of 9 over 4 plus 1 fourth plus 45 over 2 end root
    r equals square root of 100 over 4 end root
    r = 5
    Option (A) is correct.

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