Question

A= 1.2+2.3+3.4+......+ 49.50

Answer 1

A = 1.2 + 2.3 + ........+49.50

3A = 1.2.(3-0) + 2.3.(4-1)+........+49.50.(51 - 48)

3A = 1.2.3 + 2.3.4 - 1.2.3 +........ + 49.50.51 - 48.49.50

3A = 48.49.50 = 117600

A = 39200 

Answer 2

Ta có :
Gọi A=1.2+2.3+3.4+4.5+...+49.50
 A=1.2+2.3+3.4+4.5+...+49.50
 3.A=3.(1.2+2.3+3.4+4.5+...+49.50)
 3.A=1.2.3+2.3.3+3.3.4+3.4.5+...+3.49.50
 3.A=1.2.(3-0)+2.3.(3-0)+(3-0).3.4+(3-0).4.5+...+(3-0).49.50
 3.A=1.2.3-0+2.3.3-0+3.3.4-0+3.4.5-0+...+3.49.50-0
 3.A=1.2.3-0+2.3.4-1.2.3+5.3.4-2.3.4+...+49.50.51-48.49.50
 3.A=49.50.51
 A= 49.50.51/3
 A= 49.50.17.3/3
 A=49.50.17
 A=41650
Đáp số : A=41650

Answer 3

1x2+2x3+3x4+4x5+...+49x50=A
3.(1.2+2.3+3.4+4.5+...+49.50)=A.3
3.1.2+3.2.3+3.3.4+3.4.5+...+3.49.50+A.3
(3-0).1.2+(4-1).2.3+(5-2).3.4+(6-3).4.5+...+(51-48).49.50=A.3
3.1.2-0.1.2+4.2.3-1.2.3+5.3.4-2.3.4+6.4.5-3.4.5+...+51.49.50-48,49,50=A.3
51.49.50=A.3
51.49.50:3=A
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