Question

If in DABC, the line joining the circum-centre and the in-

centre is parallel to BC, then-

• A. r = R sin A
• B. R = r sin A
• C. r = R cos A
• D. R = r cos A

Answer 1

Answer: (C)

r = R cos A

Answer 2

Let S be the circum-centre and I, the in-centre of DABC. Since SI || BC, IN = SM Ž SM = r,

Also SB = R and BSM = A!

\ from DSBM, cos A = = $\frac { \mathrm { r } } { \mathrm { R } }$

Ž r = R cos A