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Question

Two circles with radii r1 and r2, r1>r22, touch each other externally. If α be the angle between direct common tangents, then

A
α=sin1(r1+r2r1r2)
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B
α=2sin1(r1r2r1+r2)
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C
α=sin1(r1r2r1+r2)
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D
α=sin1(r1r2)
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Solution

The correct option is C α=sin1(r1r2r1+r2)
From the above image.
In ΔDEC,
DEEC=sinαr2sinα=EC
Now, we see
ΔABCΔDECABDE=BCECABDE=BE+ECECABDE1=BEECr1r21=(r1+r2r2)sinαr1r2r1+r2=sinαα=sin1(r1r2r1+r2)

821639_881100_ans_1802c36490434dd48550e4112fb6ffd3.jpg

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