Tìm một số có 2 chữ số biết rằng tổng các chữ số của nó bằng 9 . nếu viết thêm chữ số 0 sen vào giữa hai chữ số của nó thì được một số mới gấp 9 lần số ban đầu.

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tìm một số có 2 chữ số biết rằng tổng các chữ số của nó bằng 9 . nếu viết thêm chữ số 0 sen vào giữa hai chữ số của nó thì được một số mới gấp 9 lần số ban đầu.
giải: giả sử số cần tìm là ab
theo đề bài ta có a + b = 9 =>b=9-a
lại có: a0b=9.ab
ó a.100+b=9.(a.10+b)
óa.100+b=90.a+9.b
ó10.a=8.b
ó10.a=8.(9-a)
ó10.a=72-8.a
ó18.a=72
=>a=72/18=4
=>b=9-a=5
Vậy số cần tìm là 45.

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