S₁₀₀ = ((2a₁ + (n - 1)·d)/2) · n = ((2·10 +99·(-3))/2) · 100 = (20 - 297)·50 = -277·50 =
= -13850
1)2/7х=28
х=28:2/7
х=28*7/2
х=98
2)9/5х=45
х=45:9/5
х=45*5/9
х=25
3)25/12х=150
х=150:25/12
х=150*12/25
х=72
№ 271.
а) 11/12 - 5/12 = 6/12 = 1/2
б) 5/14 + 2/14 = 7/14 = 1/2
в) 3 7/18 - 1 1/18 = 2 6/18 = 2 1/3
г) 4 6/15 + 3 4/15 = 7 10/15 = 7 2/3
1) 260 +9у= 530
9у= 270
У=30
2) (25-у)*6=54
25-у=9
-у = -16
у=16
3)(630:а+50)-40=100
630:а +50=140
630:а = 90
а= 7
4)62 -(116+у):17=10
(116+у):17=52
116+у= 884
у= 768