S1=8+9+10+...+19+20
S1=20+19+18+...+9+8
________________
2S1=
[tex] \frac{28(20 - 8)}{2} = \frac{28 \times 12}{2} = 28 \times 6 = 168[/tex]
S2= 36+37+38+39+40+...+ 50+51
Voi proceda că mai sus:
2S2= [(51+36)(52-36)]÷2 = (87•16)÷2 = 87•8=696
12+14+16+...+= [(100+12)(101-12)]÷2=
=(112•89)÷2=56•89=5401
