Question

diem kiem tta 1 tiet mon toan cua mot to hoc sinh duoc ghi lai o bang "tan so" sau:

diem(x)	5	6	9	10
tan so(n)	2	5	n	1

Biet diem trung binh cong bang 6,8.Hay tim gia tri cua n

Answer 1

$\overline{x} = 6,8 \iff \dfrac{5\cdot 2 + 6 \cdot 5 + 9 \cdot n + 10 \cdot 1}{2 + 5 + n + 1} = 6,8$

$\iff \dfrac{50 + 9n}{8 + n} = 6,8$

$\iff 50 + 9n = 6,8(8 + n)$

$\iff 9n - 6,8n = 6,8 \cdot 8 - 50$

$\iff 2,2n = 4,4$

$\iff n = 2$ (N)

Vậy $n = 2$