Tìmx :
(7/5+x)-2/5=4
(7/5+x) = 4+ 2/5
(7/5+x) = 22/5
x = 22/5 - 7/5
x = 3
1 và 3/4 + x - 7/8 = 5
7/4 + x -7/8 =5
x - 7/8 = 5 - 7/4
x - 7/8 = 13/4
x = 13/4 + 7/8
x = 33/8
( x - 2/3 ) : 2/5 = 1/4
( x - 2/3 ) = 1/4 x 2/5
( x - 2/3 ) = 1/10
x = 1/10 + 2/3
x = 23/30
Tính nhanh :
1/30 + 1/42 + 1/56 + 1/72 + 1/81 + 1/90 + 1/110
= ( 1/30 + 1/90 ) + ( 1/42 + 1/56 ) + ( 1/72 + 1/81 ) + 1/110
= 2/45 + 1/24 + 17/648 + 1/110
= ( 2/45 + 1/110 ) + ( 1/24 + 17/648 )
= 53/990 + 11/162
= 541/4455