1.

In the given figure, chord AB subtends an angle of `115^@` at the centre O of a circle of radius 20 cm. Calculate AB correct to nearest cm.

Answer» Let `D` is the mid point of `AB`.
Please refer to video to see the diagram.
Now, in `Delta OAD` and `Delta OBD`,
`OA = OB`(radius)
`OD = OD`
`AD = DB`
`:. Delta OAD ~= Delta OBD`
`:. /_AOD = /_BOD = 115/2 = 57.5^@`
Now, `AD = AOsin57.5^@ = 20sin57.5^@`
We know, `sin 60^@ = sqrt3/2 ~=0.86`
`:. sin57.5^@ ~= 0.8`
So, `AD = 20**0.8 = 16cm`
`:. AB = 2**AD = 2**16 = 32cm`


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