Question

Tính : S = 1*2 + 2*3 + 3*4 + ........+ 99*100

Answer 1

S = 1*2 + 2*3 + 3*4 + ........+ 99*100

=>3S=1.2.3+2.3.3+3.4.3+...+99.100.3

=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+99.100.(101-98)

=1.2.3-0.1.2+2.3.4-1.2.3+3.4.5-2.3.4+...+99.100.101-98.99.100

=99.100.101

=999900

=>S=333300

Answer 2

thank you

Answer 3

S = 1*2 + 2*3 + 3*4 + ........+ 99*100

=>3S = 1.2.3+2.3.3+3.4.3+...+99.100.3

=> 3S = 1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+99.100.(101-98)

=> 3S = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4+...+99.100.101 - 98.99.100

=> 3S =  99.100.101 = 999900

=> S=333300 

Answer 4

S = 1*2 + 2*3 + 3*4 + ........+ 99*100 

=>3S = 1.2.3+2.3.3+3.4.3+...+99.100.3 

=> 3S = 1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+...+99.100.(101-98) 

=> 3S = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4+...+99.100.101 - 98.99.100 

=> 3S =  99.100.101 = 999900 

=> S=333300