Question

[Đức Nhật Huỳnh]

cấu tính nhanh
1x2+2x3+3x4+4x5+...+49x50=
giúp minh giai bai nay vs

 

 

Answer 1

[Đức Nhật Huỳnh]

Ta có :
Gọi A=1.2+2.3+3.4+4.5+...+49.50
\Rightarrow A=1.2+2.3+3.4+4.5+...+49.50
\Rightarrow 3.A=3.(1.2+2.3+3.4+4.5+...+49.50)
\Rightarrow 3.A=1.2.3+2.3.3+3.3.4+3.4.5+...+3.49.50
\Rightarrow 3.A=1.2.(3-0)+2.3.(3-0)+(3-0).3.4+(3-0).4.5+...+(3-0).49.50
\Rightarrow 3.A=1.2.3-0+2.3.3-0+3.3.4-0+3.4.5-0+...+3.49.50-0
\Rightarrow 3.A=1.2.3-0+2.3.4-1.2.3+5.3.4-2.3.4+...+49.50.51-48.49.50
\Rightarrow 3.A=49.50.51
\Rightarrow A=49.50.51349.50.51:3
\Rightarrow A=49.50.17.3349.50.17.3:3
\Rightarrow A=49.50.17
\Rightarrow A=41650
Đáp số : A=41650