a) 8x + ( 2+5+7+12+19+31+50+81) = 447
8x + 207 = 447
8x = 447 - 207 = 240
x = 240 : 8 = 30
b) 10x + [( 21 + 3 ) x 10 : 2 ] = 335
10x + 120 = 335
10x = 335 - 120 = 215
x = 215 : 10
x = 21,5
a) ( x+3 ) + ( x+5 ) + ( x+7 ) +...+ ( x+21)= 335
<=> có 10 cặp
( x+x+x+...+x ) + ( 3+5+7+...+21 ) = 335
=> 10x + 120 = 335
10x = 335 - 120
10x = 215
x = 215 : 10
x = 21,5