1.

S.T f=(R )/(2) in the caseof a spherical mirrorwhere symbols have theirusual notations

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Solution :
From the figure `tan theta=(MD)/(CD)`
For small ANGLES `tantheta^(@)= theta `(rad)
`therefore theta("rad") =(MD)/(CD)"" `.........(1)
Also, `tan 2theta=(MD)/(FD)`
for small angles,`tan 2theta^(@)=2theta("rad")=(MD)/(FD)""`............(2)
Using (1) and (2) we can WRITE
`2((MD)/(CD))=(MD)/(FD)`
i.e. `CD=2FD`
i.e. `R=2f or f=(R)/(2)`


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