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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#