1)= S= cu Suma lui Gauss
= 10×11/2 = 5×11= 55
2) S= scoatem factori comun (2)
S= 2×(1+2+3+.....+10)
S= 2×10×11/2= 10×11=
= 110
3)= 2019 ×2020/2= 2019×1010=
= 2039190
4)= scoatem factori comun (2)
= 2×(1+2+3+......+1010)
S= 2 × 1010×1011/2=
= 1010×1011= 1021110
Sau
1010×(1010+1) = 1010² +1010
5)= scoatem factori comun (5)
S= 5×(1+2+3+......+7)
= 5× 7×8 /2 = 5×7×4= 140
6)=
78×79/2 − 10×11/2=
= 39× 79 − 5×11 =
= 3081 − 55= 3026
