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1)

If 20 m of wire is available for fencing off a flower -bed in the form of a circular sector, then the maximum area ( in sq m) of the flower-bed is 


A) 12.5

B) 10

C) 25

D) 30

Answer:

Option C

Explanation:

   Total length = 2r+rθ =20

       θ=202rr

  19112019231_tria.PNG

 Now, area of flower-bed,

          A=12r2θA=12r2(202r)r

  A=10rr2

  dAdr=102r

For maxima or minima , put  dAdr=0

102r=0r=5

  Amax=12(5)2[202(5)5]

               =12×25×2=25sq.m