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question mark

If X greater than 0 and Y greater than 0 and X+Y=18 then the minimum value of XY is
A. -1
B. 1
C. 2
D. -2

Janhavi , 5 Years ago
Grade 12
anser 1 Answers
Arun

Last Activity: 5 Years ago

Dear student
 
Let A = xy
A = x (18-x)
 
dA/dx = 18 -2x = 0
x = 9
 
Now d²A /dx² = -2 (maximum)
 
Hence maximum of A will exist when x = y = 9
 
Hence A = xy = 9*9 = 81

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