Plane Geometry


  1. The centre of a circle of radius 5 cm is ‘O’. T is an external point where OT = 13 cm and OT intersects the circle at point E. AB is a tangent at point E. What is the length of AB ?









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    On the basis of given in question , we draw a figure of a circle with centre O in which T is an external point where OT = 13 cm and OT intersects the circle at point E and AB is a tangent at point E

    ∠OPT = 90°
    In ∆OPT,
    OT² = OP² + PT²
    ⇒ 13² = 5² + PT²
    ⇒ PT² = 169 – 25 = 144
    ⇒ PT = √144 = 12 cm.
    Tangents drawn from an external point on a circle are equal.
    ∴ Let AP = AE = y
    ⇒ AT = PT – AP = (12 – y) cm
    OE ⊥ AB
    ⇒ ∠OEA = 90°
    ⇒ ∠ AET = 90°
    ∴ AT² = AE² + ET²
    ⇒ (12 – y)² = y² + (13 – 5)²
    ⇒ 144 – 24y + y² = y² + 64
    ⇒ 24y = 80

    Correct Option: B

    On the basis of given in question , we draw a figure of a circle with centre O in which T is an external point where OT = 13 cm and OT intersects the circle at point E and AB is a tangent at point E

    ∠OPT = 90°
    In ∆OPT,
    OT² = OP² + PT²
    ⇒ 13² = 5² + PT²
    ⇒ PT² = 169 – 25 = 144
    ⇒ PT = √144 = 12 cm.
    Tangents drawn from an external point on a circle are equal.
    ∴ Let AP = AE = y
    ⇒ AT = PT – AP = (12 – y) cm
    OE ⊥ AB
    ⇒ ∠OEA = 90°
    ⇒ ∠ AET = 90°
    ∴ AT² = AE² + ET²
    ⇒ (12 – y)² = y² + (13 – 5)²
    ⇒ 144 – 24y + y² = y² + 64
    ⇒ 24y = 80

    ⇒ y =
    80
    =
    10
    cm.
    243

    Similarly, BE =
    10
    cm.
    3

    ∴ AB = AE + BE =
    20
    cm.
    3


  1. In the following figure, AB is the diameter of circle and CD is a chord equal to the radius. AC and BD when extended meet at E. ∠AEB = ?










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    As per the given figure in question ,

    In ∆ OCD,
    OC = OD = CD
    ∴ ∆ OCD is an equilateral triangle.
    As we know that each angle of an equilateral triangle is 60° .
    ∴ ∠COD = 60°
    Now, ∠CBD = ( 1/2 )∠COD
    ⇒ ∠CBD = 30°
    ∠ACB is angle of semi-circle.
    ∴ ∠ACB = 90°
    ⇒ ∠ BCE = 180° – ∠ACB = 180° – 90° = 90°
    In ∆ BCE,

    Correct Option: B

    As per the given figure in question ,

    In ∆ OCD,
    OC = OD = CD
    ∴ ∆ OCD is an equilateral triangle.
    As we know that each angle of an equilateral triangle is 60° .
    ∴ ∠COD = 60°
    Now, ∠CBD = ( 1/2 )∠COD
    ⇒ ∠CBD = 30°
    ∠ACB is angle of semi-circle.
    ∴ ∠ACB = 90°
    ⇒ ∠ BCE = 180° – ∠ACB = 180° – 90° = 90°
    In ∆ BCE,
    ∠BCE = 90°, ∠CBE = ∠CBD = 30°
    ∴ ∠BCE + ∠CBE +∠CEB = 180°
    ⇒ 90° + 30° + ∠CEB = 180°
    ⇒ ∠CEB = 180° – 120° = 60°
    ⇒ ∠ AEB = 60°



  1. In the following figure, ∠ ABC = 69°,∠ ACB = 31° then∠ BDC = ?










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    On the basis of given figure in question ,

    Given that , ∠ ABC = 69°,∠ ACB = 31°
    In ∆ ABC,
    As we know that ,
    ∠BAC + ∠ABC + ∠ACB = 180°
    ⇒ ∠BAC + 69° + 31° = 180°

    Correct Option: A

    On the basis of given figure in question ,

    Given that , ∠ ABC = 69°,∠ ACB = 31°
    In ∆ ABC,
    As we know that ,
    ∠BAC + ∠ABC + ∠ACB = 180°
    ⇒ ∠BAC + 69° + 31° = 180°
    ⇒ ∠ BAC = 180° – 100° = 80°
    Since angles in the same segment are equal.
    ∴ ∠BDC = 80°


  1. Two circles whose radii are 10 cm and 8 cm, intersect each other and their common chord is 12 cm long. What is the distance between their centres ?









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    According to question , we draw a figure of two circles whose radii are 10 cm and 8 cm, intersect each other and their common chord is 12 cm long

    OP = 10 cm, O’P = 8 cm and PQ = 12 cm ;

    PL =
    1
    PQ = 6 cm
    2

    In right angled ∆ OLP,
    OL = √OP² - LP²
    OL = √10² - 6² = √64 = 8 cm

    Correct Option: D

    According to question , we draw a figure of two circles whose radii are 10 cm and 8 cm, intersect each other and their common chord is 12 cm long

    OP = 10 cm, O’P = 8 cm and PQ = 12 cm ;

    PL =
    1
    PQ = 6 cm
    2

    In right angled ∆ OLP,
    OL = √OP² - LP²
    OL = √10² - 6² = √64 = 8 cm
    In ∆ O'LP,
    O'P² = O'L² + LP²
    ⇒ O'L = √O'P² - LP²
    O'L = √8² - 6² = √28 = 5.29 cm
    ∴ OO' = OL + LO' = 8 + 5.29 = 13.29 cm



  1. In any triangle ABC, AD, BE and CF are medians. What is the relation between the perimeter of triangle and sum of all three medians ?









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    On the basis of given in question , we draw a figure of triangle ABC whose AD, BE and CF are medians

    We know that the sum of any two sides of a triangle is greater than twice the median bisecting the third side.
    AD is the median on BC.
    ∴ AB + AC > 2AD .....( 1 )
    Similarly,
    AB + BC > 2BE .....( 2 )
    BC + AC > 2CF .....( 3 )
    On adding all three equations, we get

    Correct Option: B

    On the basis of given in question , we draw a figure of triangle ABC whose AD, BE and CF are medians

    We know that the sum of any two sides of a triangle is greater than twice the median bisecting the third side.
    AD is the median on BC.
    ∴ AB + AC > 2AD .....( 1 )
    Similarly,
    AB + BC > 2BE .....( 2 )
    BC + AC > 2CF .....( 3 )
    On adding all three equations, we get
    AB + AC + AB + BC + BC + AC > 2AD + 2BE + 2CF
    ⇒ 2 (AB + BC + AC) > 2 (AD + BE + CF)
    ⇒ AB+BC+AC > AD + BE + CF