选一选

当x=7时，7x-2540，  7x+2540

$\frac {1} {2}+2$$\frac {1} {2}\times 2$

-0.2-0.3

$\frac {1} {2}$-2

-22

-1-2

2.5-2.5

$\frac {1} {2}$$-\frac {1} {2}$

-20