78. Thực hiện phép tính
a) 36.68 + 68.64 = 2448 + 4352 = 6800
b) 39.113 + 87.39 = 4407 + 3393 = 7800
c) 23.15 + 23.35 = 120 + 280 = 400
d) [600 - (40 : 23 + 3.53)] : 5 = [600 - (5 + 375)] : 5 = [600 - 380] : 5 = 220 : 5 = 44
79. Tìm số tự nhiên x, biết
a) (x - 35) - 120 = 0
(x - 35) = 0 + 120
(x - 35) = 120
x = 120 + 35
x = 155
b) 310 - (118 - x) = 217
(118 - x) = 310 - 217
(118 - x) = 93
x = 118 - 93
x = 25
c) 156 - (x + 61) = 82
(x + 61) = 156 - 82
(x + 61) = 74
x = 74 - 61
x = 13
d) 814 - (x - 305) = 712
(x - 305) = 814 - 712
(x - 305) = 102
x = 102 + 305
x = 407
e) 2x -138 = 23.32
2x - 138 = 72
2x = 72 + 138
2x = 210
x = 210 : 2
x = 105
g) 5(x + 35) = 515
(x + 35) = 515 : 5
(x + 35) = 103
x = 103 - 35
x = 68
79.a, x-35=0+120=> x= 120+35= 155
b, 118-x= 310-217=> 118-x= 33 => x= 118-33=85
c, x+61= 156-82=> x+61= 74 => x= 74-61= 13
d, x-305= 814-712=> x-305=102=> x=102+305= 4 07
\(78,\)
\(a,36.68+68.64=\left(36+64\right)68=100.68=6800\)
\(b,39.113+87.39=39\left(113+87\right)=39.200=7800\)
\(c,2^3.15+2^3.35=2^3\left(15+35\right)=2^3.50=400\)
\(d,\left[600-\left(40:2^3+3.5^3\right)\right]:5\)
\(=\left[600-\left(5+375\right)\right]:5\)
\(=\left[600-380\right]:5\)
\(=220:5\)
\(=44\)
___________________________________________
\(79,\)
\(a,\left(x-35\right)-120=0\)
\(x-35=0+120\)
\(x-35=120\)
\(x=120+35\)
\(x=155\)
\(b,310-\left(118-x\right)=217\)
\(118-x=310-217\)
\(118-x=93\)
\(x=118-93\)
\(x=25\)
\(c,156-\left(x+61\right)=82\)
\(x+61=156-82\)
\(x+61=74\)
\(x=74-61\)
\(x=13\)
\(d,814-\left(x-305\right)=712\)
\(x-305=814-712\)
\(x-305=102\)
\(x=102+305\)
\(x=407\)
\(e,2x-138=2^3.3^2\)
\(2x-138=72\)
\(2x=72+138\)
\(2x=210\)
\(x=210:2\)
\(x=105\)
\(g,5\left(x+35\right)=515\)
\(x+35=515:5\)
\(x+35=103\)
\(x=103-35\)
\(x=68\)
\(h,20-\left[7\left(x-3\right)+4\right]=2\)
\(\left[7\left(x-3\right)+4\right]=20-2\)
\(\left[7\left(x-3\right)+4\right]=18\)
\(7\left(x-3\right)=18-4\)
\(7\left(x-3\right)=14\)
\(x-3=14:7\)
\(x-3=2\)
\(x=2+3\)
\(x=5\)
\(i,\left[\left(6x-39\right):3\right].28=5628\)
\(\left[\left(6x-39\right):3\right]=5628:28\)
\(\left[\left(6x-39\right):3\right]=201\)
\(6x-39=201.3\)
\(6x-39=603\)
\(6x=603+39\)
\(6x=642\)
\(x=642:6\)
\(x=107\)
\(78,\)
\(a,36.68+68.64=68\left(36+64\right)=68.100=6800\)
\(b,39.113+87.39=39\left(113+87\right)=39.200=7800\)
\(c,2^3.15+2^3.35=2^3\left(15+35\right)=2^3.50=8.50=400\)
\(d,\left[600-\left(40:2^3+3.5^3\right)\right]:5\)
\(=\left[600-\left(5+375\right)\right]:5\)
\(=\left[600-380\right]:5\)
\(=220:5\)
\(=44\)
___________________________________
\(79,\)
\(a,\left(x-35\right)-120=0\)
\(x-35=0+120\)
\(x-35=120\)
\(x=120+35\)
\(x=155\)
\(b,310-\left(118-x\right)=217\)
\(118-x=310-217\)
\(118-x=93\)
\(x=118-93\)
\(x=25\)
\(c,156-\left(x+61\right)=82\)
\(x+61=156-82\)
\(x+61=74\)
\(x=74-61\)
\(x=13\)
\(d,814-\left(x-305\right)=712\)
\(x-305=814-712\)
\(x-305=102\)
\(x=102+305\)
\(x=407\)
\(e,2x-138=2^3.3^2\)
\(2x-138=8.9\)
\(2x-138=72\)
\(2x=72+138\)
\(2x=210\)
\(x=210:2\)
\(x=105\)
\(g,5\left(x+35\right)=515\)
\(x+35=515:5\)
\(x+35=103\)
\(x=103-35\)
\(x=68\)
\(h,20-\left[7\left(x-3\right)+4\right]=2\)
\(\left[7\left(x-3\right)+4\right]=20-2\)
\(\left[7\left(x-3\right)+4\right]=18\)
\(7\left(x-3\right)=18-4\)
\(7\left(x-3\right)=14\)
\(x-3=14:7\)
\(x-3=2\)
\(x=2+3\)
\(x=5\)
\(i,\left[\left(6x-39\right):3\right].28=5628\)
\(\left[\left(6x-39\right):3\right]=5628:28\)
\(\left[\left(6x-39\right):3\right]=201\)
\(\left(6x-39\right)=210.3\)
\(\left(6x-39\right)=603\)
\(6x=603+39\)
\(6x=642\)
\(x=642:6\)
\(x=107\)
