Double sums!

Algebra Level 2

a = 1 4 b = 0 4 a b = ? \large \sum_{a=1}^4 \sum_{b=0}^4 a^b = \, ?


The answer is 498.

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1 solution

Kay Xspre
Mar 1, 2016

The most simple way is the rewrote b b summation as 1 + a + a 2 + a 3 + a 4 = a 5 1 a 1 1+a+a^2+a^3+a^4 = \frac{a^5-1}{a-1} . This formula is valid for a 1 a \neq 1 . As the summation requires a = 1 a = 1 , then the first term of a a summation shall be 5. The rest is 5 + a = 2 4 a 5 1 a 1 = 5 + 31 + 242 2 + 1023 3 = 498 5+\sum_{a=2}^4 \frac{a^5-1}{a-1} = 5+31+\frac{242}{2}+\frac{1023}{3} = 498

Hello @Kay do you know where i could learn the theorem of double sum(like the question stated above)?

Victor Zhang - 5 years, 3 months ago

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Usually I don't have particulars, but I expand the summation. This may be done if it is not complicated, but if it is, then we will have to look for other way. Alternatively for this question, one can simply expand the a a summation first and make it to b = 0 4 ( 1 b + 2 b + 3 b + 4 b ) = 4 + 10 + 30 + 100 + 354 = 498 \sum_{b=0}^4(1^b+2^b+3^b+4^b) = 4+10+30+100+354 = 498

Kay Xspre - 5 years, 3 months ago

From where u got the formula of eleminating b

uday kiran - 5 years, 3 months ago

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None. You just have to distribute the B sigma into a 0 + a 1 + a 2 + a 3 + a 4 a^0+a^1+a^2+a^3+a^4 . The rest depends on how you approach this A sigma

Kay Xspre - 5 years, 3 months ago

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