An algebra problem by prasanth pp

Algebra Level 3

If e f ( x ) = ( 10 + x ) / ( 10 x ) e^{f(x)}=(10+x)/(10-x) and f ( x ) = k × f ( 200 x / ( 100 + x 2 ) ) f(x)=k \times f(200x/(100+x^2)) find the value of k k :

0.216 0.125 0.5 0.6 0.7

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1 solution

Prasanth Pp
Mar 24, 2016

Given e f ( x ) = ( 10 + x ) / ( 10 x ) e^f(x)=(10+x)/(10-x)
taking natural log on both sides..> f ( x ) = l n ( 10 + x ) / ( 10 x f(x)=ln(10+x)/(10-x )...............(1)
also given f ( x ) = k × f ( 200 x / ( 100 + x 2 ) ) f(x) = k \times f(200x/(100+x^2)) ...............................(2)
equating both equations:
f ( x ) = l n ( 10 + x ) / ( 10 x ) = k × l n ( 10 + 200 x / ( 100 + x 2 ) / 10 200 x / ( 100 + x 2 ) f(x)=ln(10+x)/(10-x)= k \times ln(10+ 200x/(100+x^2)/10-200x/(100+x^2)
l n ( 10 + x ) / ( 10 x ) = k × l n ( 10 x 2 + 200 x + 1000 ) / ( 10 x 2 200 x + 1000 ) ln(10+x)/(10-x)= k \times ln(10x^2+200x+1000)/(10x^2-200x+1000)
l n ( 10 + x ) / ( 10 x ) = k × l n ( x 2 + 20 x + 100 ) / ( x 2 20 x + 100 ) ln(10+x)/(10-x)= k \times ln(x^2+20x+100)/(x^2-20x+100)
l n ( 10 + x ) / ( 10 x ) = k × l n ( ( 10 + x ) / ( 10 x ) ) 2 ln(10+x)/(10-x)= k \times ln((10+x)/(10-x))^2
l n ( 10 + x ) / ( 10 x ) = k × 2 l n ( 10 + x ) / ( 10 x ) ln(10+x)/(10-x)= k \times 2 ln(10+x)/(10-x)
=>2*k=1
k=1/2
k=0.5

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