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If x and y be two real variables such that x>0 and xy=1. Then, find the minimum value of x+y.
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Solution:-As we know that, according to the concept of Maxima and minima of diffrentiation..
if f(x) is any equation......
then,if
f'(x)=0
and ,f''(x)>0,then that value of x will give maximum value but if we get f''(x)<0 ,then the value of x will give minimum value
now here
xy=1
y=1/x
so, putting this value in x+y and taking it as f(x) we can write
f(x)=x+1/x=x+x^-1
diffrentiating with respect to x we get
f'(x)=1+(-1)x^-2
f'(x)=1-1/x^2
clearly we can say that,f(1)=0
f(1)=1-1=0
diffrentiating it again
f''(x)=0-(-2)/x^3=2/x^3
f''(1)=2/1=2
so,from here we are getting
f'(1)=0 and f''(1)>0
so,x=1 will give minimum value
x+y=x+1/x=1+1/1=2=minimum value