Answers

2015-03-26T19:57:19+05:30
  Initial dia. = d 
Initial r = d/2 
Initial S.A. = 4*Pi*(d^2)/4 

New dia. = d-(25% of d) 
= d-(d/4) = 3d/4 

New r = 3d/8 

Thus, New S.A. = 4*Pi*(9d^2)/64 

Now %age increase = [ (New S.A.)/(Initial S.A.) ] * 100 
= (9/64) * 4 * 100 
= 56.25 % 

So, %age decrease = 100 - 56.25 = 43.75 % 
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2015-03-26T20:12:05+05:30
We know, CSA sphere=4πr²=4π(d/2)²=πd² (radius= diameter/2)
diam. decreased by 25%
i.e., (100-25)%=75% of the diam. is left
Therefore, 75/100 of diam.=3diam./4= 3d/4 × 1/2= 3d/8 (r=d/2)
NEW CSA, 
4π{(3d/4)/2}²
4π(3d/8)²=4π×9d²/64
=9πd²/16
Now, decrease%= decrease in CSA × 100%/ original CSA
= (Original CSA-new CSA)×100/ Original CSA
={(πd²-9πd²/16) × 100}/πd²
=7πd²/16 × 1/πd² × 100= 43.75 % decrease in CSA

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