Question

1) Tính tổng:

A = 1.2 +2.3 + 3.4 + ... + 19.20

2)Tìm số nguyên x, y biết:

xy - x + 2y = 3

Answer 1

a) Ta có :

\(A=1.2+2.3+3.4+.....+19.20\)

\(3A=1.2.\left(3-0\right)+2.3.\left(4-1\right)+3.4.\left(5-2\right)+.............+19.20.\left(21-18\right)\)

\(3A=1.2.3+2.3.4-1.2.3+3.4.5+-2.3.4+................+19.20.21-18.19.20\)\(3A=\left(1.2.3+2.3.4+.............+19.20.21\right)-\left(1.2.3+2.3.4+..........+18.19.20\right)\)\(3A=19.20.21=7980\)

\(\Rightarrow A=2660\)

~ Chúc bn học tốt ~

Answer 2

xy - x + 2y = 3

\(\Rightarrow\) x(y - 1) + 2y = 1 + 2

\(\Rightarrow\) x(y - 1) + 2y - 2 = 1

\(\Rightarrow\) x(y -1) + 2(y - 1) =1

\(\Rightarrow\) (x + 2) . (y - 1) = 1

Ta có bảng sau:

x + 2	y - 1
1	1
- 1	- 1

Suy ra

x	y
-1	2
-3	0