Binomial terms count

Algebra Level 3

Find the number of terms in the expansion of ( 1 + x ) 101 ( 1 x + x 2 ) 100 (1+x)^{101}(1-x+x^2)^{100}


The answer is 202.

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1 solution

Sathvik Acharya
Dec 21, 2020

Using the well known identity , x 3 + 1 = ( x + 1 ) ( x 2 x + 1 ) x^3+1=(x+1)(x^2-x+1) , ( x + 1 ) 101 ( x 2 x + 1 ) 100 = ( x 3 + 1 ) 100 ( x + 1 ) = x ( x 3 + 1 ) 100 + ( x 3 + 1 ) 100 \begin{aligned}(x+1)^{101}(x^2-x+1)^{100} &=(x^3+1)^{100}(x+1) \\ &= x(x^3+1)^{100}+(x^3+1)^{100} \end{aligned} From the binomial theorem , we have, ( x 3 + 1 ) 100 = ( 100 0 ) x 0 + ( 100 1 ) x 3 + ( 100 2 ) x 6 + + ( 100 99 ) x 297 + ( 100 100 ) x 300 \; \;\;\;\;\;\;\;\;\;\;(x^3+1)^{100}=\binom{100}{0}x^0+\binom{100}{1}x^3+\binom{100}{2}x^6+\cdots+\binom{100}{99}x^{297}+\binom{100}{100}x^{300} x ( x 3 + 1 ) 100 = ( 100 0 ) x 1 + ( 100 1 ) x 4 + ( 100 2 ) x 7 + + ( 100 99 ) x 298 + ( 100 100 ) x 301 \implies x(x^3+1)^{100}=\binom{100}{0}x^1+\binom{100}{1}x^4+\binom{100}{2}x^7+\cdots+\binom{100}{99}x^{298}+\binom{100}{100}x^{301} Observe that ( x 3 + 1 ) 100 (x^3+1)^{100} and x ( x 3 + 1 ) 100 x(x^3+1)^{100} have no common powers of x x in their expansion, so their sum cannot be simplified any further (the terms cannot be combined). Both of the above expansions have 101 101 terms. Therefore, the total number of terms is 202 \boxed{202}

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