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Question
In the given figure, O is the centre of the two concentric circles of radii 4 cm and 6cm respectively. AP and PB are tangents to the outer and inner circle respectively. If PA = 10cm, find the length of PB up to one place of the decimal.
Solution
Given, O is the center of two concentric circles of radii OA = 6 cm and OB = 4 cm.
PA and PB are the two tangents to the outer and inner circles respectively and PA = 10 cm.
Now, tangent drawn from an external point is perpendicular to the radius at the point of contact.
∴∠OAP = ∠OBP = 90°
∴ From right – angled ΔOAP,OP2 = OA2 + PA2
`⇒OP = sqrt(OA^2 + PA^2)`
`⇒ OP = sqrt (6^2 +10^2)`
`⇒ OP = sqrt( 136cm)`.
∴ From right – angled `Δ OAP,OP^2 = OB^2 + PB^2`
`⇒ PB = sqrt(OP^2 -OB^2)`
`⇒ PB = sqrt(136-16)`
`⇒ PB= sqrt(120cm)`
⇒ PB = 109.cm.
∴ The length of PB is 10.9 cm.
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