Show that 0.001728 is the cube of a rational number. Find that r

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19. Show that 0.001728 is the cube of a rational number. Find that rational number whose cube is 0.0017288.


We have

space space space space 0.001728 space equals space 1728 over 1000000

Now,  
 
We haveNow,   
∴                               

∴    space space space space 1728 over 1000000 space equals space fraction numerator 2 cross times 2 cross times 2 cross times 2 cross times 2 cross times 2 cross times 3 cross times 3 cross times 3 over denominator 2 cross times 2 cross times 2 cross times 2 cross times 2 cross times 2 cross times 5 cross times 5 cross times 5 cross times 5 cross times 5 cross times 5 end fraction

                        space space space space equals space fraction numerator 3 cross times 3 cross times 3 over denominator 5 cross times 5 cross times 5 cross times 5 cross times 5 cross times 5 end fraction

                        space space equals fraction numerator 3 cubed over denominator 5 cubed cross times 5 cubed end fraction

                   space space space space space space space equals open parentheses fraction numerator 3 over denominator 5 cross times 5 end fraction close parentheses cubed

∴   space space space space space space space space space space space space space space cube root of 1728 over 1000000 end root equals space open parentheses fraction numerator 3 over denominator 5 cross times 5 end fraction close parentheses cubed

space space space space space space space rightwards double arrow space space space space space space space cube root of 1728 over 1000000 end root equals fraction numerator 3 over denominator 5 cross times 5 end fraction

     rightwards double arrow space space space space space space cube root of 0.001728 end root space equals space 3 over 25

Thus, 0.001728 is the cube of space 3 over 25.

 

 
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