Question

Tính  A= 1 * 2 + 2 * 3 + 3 * 4 + 4 * 5 + ... + 49 * 50

Answer 1

A=1.2+2.3+3.4+4.5+...+49.50

=>3A=1.2.3+2.3.3+3.4.3+4.5.3+...+49.50.3

        =1.2.3+2.3(4-1)+3.4(5-2)+4.5(6-3)+...+49.50(51-48)

        =1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+49.50.51-48.49.50

        =49.50.51

        =124950

<=>A=41650

Vậy A=41650

Answer 2

A=1.2+2.3+3.4+4.5+...+49.50

=>3A=1.2.3+2.3.3+3.4.3+4.5.3+...+49.50.3

=1.2.3+2.3(4-1)+3.4(5-2)+4.5(6-3)+...+49.50(51-48)

=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+49.50.51-48.49.50

=49.50.51=124950

=>A=41650

vậy A=41650