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out of two concentric circles ,the …

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out of two concentric circles ,the radius of outer circle is 5cm and the chord AC of length 8cm is a tangent to inner circle . find the radius of the inner circle
  • 1 answers

Sia ? 6 years, 6 months ago


Since AC is a tangent to the inner circle.
{tex}\angle OBC = 90^\circ {/tex}
AC is a chord of the outer circle.
We know that, the perpendicular drawn from the centre to a chord of a circle, bisects the chord.
{tex}AC = 2BC \\ 8=2BC ​​​​​​​{/tex}
{tex}\Rightarrow{/tex} BC = 4 cm
In {tex}\Delta{/tex}OBC,
By Pythagoras theorem,
OC2 = OB2 + BC2
{tex}\Rightarrow{/tex} 52 = r2 + 42
{tex}\Rightarrow{/tex} r2 = 52 - 42
{tex}\Rightarrow{/tex} r2 = 25 - 16
{tex}\Rightarrow{/tex} r2 = 9 cm
{tex}\Rightarrow{/tex} r = 3 cm

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