Sn = (a1+ an)*n/2
S50 = (-3 + 43) * 50/2 = 1000.
1. 2,5×21=x×10,5
52,5=10.5x
10,5x=52,5
x=52,5:10,5
x=5
2. 3,1x-1,8x=-40,2+0,55
1,3x=-39,65
x=-39,65:1,3x
x=-30,5
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144/299
1) 5×5×5+-8×-8=125+-64=-61
2)8-9=-1×-1×-1=-1
3)21/2×21/2-11/4×11/4=441/4-121/16=1764/16-121/16=1643/16=102 целых 11/16
20+5х=0
5х=0-20
5х=-20
х=-20:5
х=-4