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I can not understand how to solve it .

Can anybody provide me a hint .

Added: There solution is also given but I could not understand it .

solution

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    What is the source? – 2017-02-24
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    @labbhattacharjee it came in my test – 2017-02-24
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    @labbhattacharjee I have provided the solution , may be this is useful – 2017-02-24
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    Please try avoid pictures of text or formula and replace them with typeset mathematics. You can learn about the binomial theorem. It explains which coefficients each term of $(a+b)^n$ gets. – 2017-04-14

2 Answers 2

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Try multiplying out some terms and ignoring those where the exponent is too big. For example you could start with

$(1+x)(1+2x^2+x^4)(1+3x^3+3x^6+\cdots)(1+4x^4+\cdots)(1+5x^5+\cdots)(1+6x^6+\cdots)$

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    Was doing the same, but I expect something better exists. – 2017-02-24
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    @labbhattacharjee: [OEIS A026007](http://oeis.org/A026007) does not suggest to me a simple shortcut – 2017-02-24
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HINT

Multipliers over 6 does not change coefficient of $x^6$