Question

Cho S =(1=2+3+...+97+98+99):5. Tính giá trị của S.

Answer 1

\(S=\left(1+2+3+...+99\right):5\)

\(S=\frac{\left[\left(99-1\right)+1\right]}{2}.\left(99+1\right):5\)

\(S=\left(\frac{99}{2}.100\right):5\)

\(S=99.50:5\)

\(S=4950:5\)

\(S=990\)

Answer 2

S = ( 1 + 2 + 3 + .......+ 97 + 98 + 99 ) : 5

S x 5 = 1 + 2 + 3 +......+ 98 + 99

S x 5 = ( 99 + 1) + ( 98 + 2) + ( 97+3 )+...........+ ( 51 + 49 ) + 50

S x 5 = 100 + 100 + 100 + 100 +....+ 100 + 50

                 có 49 số 100                         

S x 5 = 100 x 49 + 50

S x 5 = 4900 + 50

S x 5 = 4950

S= 4950 : 5

S = 990