选一选当x=8时,9x+7x128, 9x+7128
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=
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选一选当x=2.5时,5x+11x40, 5x+1140
选一选
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