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3 12 30
9 18
9
2A=9-->A=9/2
3A+B=9--->B=18/2-27/2=-9/2
A+B+C=3--->C=3
Sn=An²+Bn+C
Sn=½n(9n-9)+3
S18=½18(9.18-9)+3
S18=9(153)+3
S18=1380
b).
-32 -70 -118
-38 -48
-10
2A=-10-->A=-10/2=-5
3A+B=-38---->B=-38+15=-23
A+B+C=-32--->C=-32+5+23=-4
Sn=An²+Bn+C
Sn=-n(5n+23)-4
S20=-20(5.20+23)-4
S20=-20(123)-4
S20=-2464