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Answer 1 · 2017-12-10T17:12:16

Because summations are linear we can move the 6 outside:

$sum_(i=7)^26 6i = 6sum_(i=7)^26 i$

The summation from 7 to 26 is the same as the summation from 1 to 26 minus the summation from 1 to 6:

$sum_(i=7)^26 6i = 6(sum_(i=1)^26 i- sum_(i=1)^6 i)$

We know that the summation of 1 to n is $(n(n+1))/2$

$sum_(i=7)^26 6i = 6((26(27))/2- (6(7))/2)$

$sum_(i=7)^26 6i = 6(330)$

$sum_(i=7)^26 6i = 1980$