Binomial coefficients and Pascal's triangle

For what value of kk is the binomial coefficient of akb5ka^{k}b^{5-k} in the expansion of (a+b)5(a+b)^5 equals 1010?
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Which of these constructions define Pascal's triangle?
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What does the summation of a row of Pascal's triangle represent?
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If we add the values of second column third row in pascal's triangle for (a+b)n(a+b)^n and (b+c)n(b+c)^n, we get ______
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How can an element of Pascal's triangle be derived using existing elements?
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