OG详解-OG7 数学1 Q34

The correct answer is 3. It’s given that y = 9x+12. Substituting 9x+ 12 for y in the second equation in the system, x+7y = 20, yields x+7(9x+12) = 20, which gives x+63x+84 = 20, or 64x+84 = 20. Subtracting 84 from each side of this equation yields 64x = -64. Dividing each side of this equation by 64 yields x =-1. Substituting -1for x in the first equation in the system, y = 9x+12, yields y = 9(-1)+12, or y = 3. Therefore, the value of y is 3.