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Question

In a ABC, the lengths of the bisectors of the angle A,B and C are x,y,z respectively. Show that 1xcosA2+1ycosB2+1zcosC2=1a+1b+1c. Also show that ab+c=1x2bc

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Solution

In ABC,x=2bcb+ccosA21xcosA2=12b+12c

similarly 1ycosB2=12c+12a,1zcosC2=12a+12b

1xcosA2+1ycosB2+1zcosC2=12b+12c+12c+12a+12a+12b=1a+1b+1c

1x2bc=14bc(b+c)2cos2A2=b2+c22bccosA(b+c)2=ab+c

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