Question

Giá trị của tổng \(S=1.2+2.3+3.4+...+49.50\).Vậy : \(S=?\)  .

Answer 1

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=\(\frac{49.50.51}{3}\)49.50.513
 A=\(\frac{49.50.17.3}{3}\)49.50.17.33
 A=49.50.17
 A=41650
Đáp số : A=41650

Answer 2

3S=1.2.3+2.3.(4-1)+3.4.(5-2)+...+49.50.(51-48)

=1.2.3-1.2.3+2.3.4-2.3.4+3.4.5-...-48.49.50+49.50.51

=49.50.51

=124950

Answer 3

3S=1.2.3+2.3.3+3.4.3+...+49.50.3

=1.2.3+2.3.(4-1)+3.4.(5-2)+...+48.49.(50-47)+49.50.(51-48)

=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+48.49.50-47.48.49+49.50.51-48.49.50

=(1.2.3-1.2.3)+(2.3.4-2.3.4)+...(47.48.49-47.48.49)-(48.49.50-48.49.50)+49.50.51

=0+0+...+0+0+49.50.51

=49.50.51

S=(49.50.51)/3

=41650

Đáp số:41650

Answer 4

3 lay o dau ra vay ban ?

Answer 5

41650

Answer 6

đây là toán lớp 5 học lại mà