Question

S=1.2+2.3+3.4+.............+n(n+1)

Answer 1

3s=1.2(3-0)+2.3(4-1)+...+n.(n+1).(n+2)-(n-1)

3a=1.2.3+2.3.4+3.4.5+....+(n-1).n(n+1)+n(n+1)(n+2)-

0.1..2-1.2.3-3.4.5-....-(n-1)n(n+1)

3a=n.(n+1)(n+2)

a=n(n+1)(n+2)/3

Answer 2

1.2+2.3+3.4.....+n.(n+1)=A 
ta có 
3.A=1.2.(3-0)+2.3.(4-1)+3.4.(5 -2)...+ n.(n+1) . ((n+2) - (n-1)) 
3.A=1.2.3+2.3.4+3.4.5+...+ (n-1) . n. (n+1)+ n. (n+1). (n+2) - 
0.1.2 -1.2.3 -2.3.4 -3.4.5 -...(n-1)n(n+1) 
3A=n.(n+1).(n+2) 
A=n.(n+1).(n+2)\3 

ai mình nt mik lại

Answer 3

k zồi , k tui