Just Think Part#2

Algebra Level 4

1 8 3 + 7 3 + 3 × 18 × 7 × 25 3 6 + 6 × 243 × 2 + 15 × 81 × 4 + 20 × 27 × 8 + 15 × 9 × 16 + 6 × 3 × 32 + 64 = a {\dfrac{18^{3} + 7^{3} + 3 \times 18 \times 7 \times 25}{3^{6} + 6 \times 243 \times 2 + 15 \times 81 \times 4 + 20 \times 27 \times 8 + 15 \times 9 \times 16 + 6 \times 3 \times 32 + 64} = a}

Find the value of [ 1439 a ] \Big[\frac{1439}{a}\Big]

Note- Don't use calculator


The answer is 1439.

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1 solution

Aadhitiya Vs
Nov 1, 2014

We Observe that - 18 ^ 3 + 7^3 + 3 * 18* 7*25 is in the form of (a+b)^3 that is (25)^3 in the denominator we see the term 3 ^ 6 and 2 ^ 6 ie. 64 hence the denominator is in the form( 2+3)^6 = 5^6

(25)^3/(5)^6 = 5 ^2^3 / 5 ^6 = 5^6/5^6 which implies a = 1

therefore 1439/1 = 1439

You can't just say that the denominator will be in that form just because 2 6 2^6 and 3 6 3^6 are there.

The denominator comes in that form because it is in the form of the expansion of ( 2 + 3 ) 6 (2+3)^6 using binomial theorem according to which, ( x + y ) n = k = 0 n ( ( n k ) x n k y k ) \displaystyle (x+y)^n=\sum_{k=0}^n\bigg( \binom{n}{k} x^{n-k}y^k \bigg)

Prasun Biswas - 6 years, 6 months ago

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Yes, I agree with you. It does need this full examination..

Niranjan Khanderia - 6 years, 6 months ago

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