If the cube root of 9 3 + 1 1 2 is of the form a b + c d , where a , b , c , d are positive integers with b > d . Find a 1 + b 2 + c 3 + d 4 .
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9 3 + 1 1 2 = 3 3 + 9 2 + 6 3 + 2 2 = ( 3 ) 3 + 3 ∗ ( 3 ) 2 ∗ 2 + 3 ∗ 3 ∗ ( 2 ) 2 + ( 2 ) 3 = ( 3 + 2 ) 3 = ( a b + c d ) 3 a 1 + b 2 + c 3 + d 4 = 1 1 + 3 2 + 1 3 + 2 4 = 2 7
Excellent innovative solution.
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Thank you.
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How much time did it take for you to figure it out?
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@Vishwak Srinivasan – Well, I am very slow in all my activity! However with this it took no time! ONLY some such things just flashes out in no time.
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Let the given number be denoted as n 3
Hence,
n 3 = 9 3 + 1 1 2
Taking 3 3 common from the R.H.S of the above equation,
n 3 = 3 3 × ( 3 + 3 1 1 3 2 )
Implies,
n = 3 × ( 3 + 3 1 1 3 2 ) 3 1
Consider the number ( 3 + 3 1 1 3 2 ) 3 1 as x + y
Therefore,
x + y = ( 3 + 3 1 1 3 2 ) 3 1 ......... (1)
x − y = ( 3 − 3 1 1 3 2 ) 3 1 ...........(2)
Multiplying (1) and (2),
x 2 − y = ( 9 − 9 1 2 1 × 3 2 ) 3 1
x 2 − y = ( 2 7 1 ) 3 1
x 2 − y = 3 1 .......... (3)
Cubing equation (1) (which I have not shown here) and comparing the rational parts of the L.H.S and R.H.S,
x 3 + 3 x y = 3 .......... (4)
Equating (3) and (4) (which results in a cubic equation in x ), we get,
x = 1 and y = 3 2
Now back to the initial steps,
n = 3 × ( x + y )
n = 3 × ( 1 + 3 2 )
n = 3 + 2
a = 1 , b = 3 , c = 1 , d = 2
Required answer is 1 1 + 3 2 + 1 3 + 2 4 = 2 7