Sn=[2a₁+(n-1)r]/2 ×n
518=[2×50+(14-1)r]/2×14/:7
74=100+13r
13r=74-100
13r=-26
r=-26:13
r=-2
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Sn=[2a₁+(n-1)r]/2 ×n
518=[2×50+(14-1)r]/2×14/:7
74=100+13r
13r=74-100
13r=-26
r=-26:13
r=-2