Question

Find the sum of the summation of 3 i minus 15, from i equals 2 to 7.

options:
9
0
−9
6

Answer 1

i think the answer would be -9

Answer 2

Answer with explanation:

`\sum_(i=2)^(i=7)3i-15\\\\=(3*2-15)+(3*3-15)+(3*4-15)+(3*5-15)+(3*6-15)+(3*7-15)\\\\=3*2+3*3+3*4+3*5+3*6+3*7-(15+15+15+15+15+15)\\\\=3*(1+2+3+4+5+6+7)-3-90\\\\=3*(7*(1+7))/(2)-93\\\\=3*28-93\\\\=84-93\\\\=-9\\\\ \text{Sum of n terms of an A.P}=(n*(a+l))/(2)`

where a, is first term and l is last term.

Option C= -9