Plane Geometry
- 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 = 80Correct 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. 24 3 Similarly, BE = 10 cm. 3 ∴ AB = AE + BE = 20 cm. 3
- 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°
- 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°
- 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
- 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 getCorrect 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