Talk:1989 AIME Problems/Problem 8

Solution 7

Subtracting the first equation from the second equation we get $3x_1+5x_2+7x_3+9x_4+11x_5+13x_6+15x_7=11$. Subtracting the second equation from the third equation gives $5x_1+7x_2+9x_3+11x_4+13x_5+15x_6+17x_7=111$. Subtracting these two equations, we get that $x_1+x_2+x_3+x_4+x_5+x_6+x_7=50$. Note that we are trying to find \[16x_1+25x_2+36x_3+49x_4+64x_5+81x_6+100x_7\] \[=(9x_1+16x_2+25x_3+36x_4+49x_5+64_6+81x_7)+(7x_1+9x_2+11x_3+13x_4+15x_5+17x_6+19x_7)\] \[=123+(5x_1+7x_2+9x_3+11x_4+13x_5+15x_6+17x_7+2x_1+x_2+x_3+x_4+x_5+x_6+x_7)\] \[=123+111+100=\boxed{334}\]

-Heavytoothpaste