当x=0.5,y=0.9时,求2.5x-2y+5.5x的值

题目

当x=0.5,y=0.9时,求2.5x-2y+5.5x的值
先化简再求值

答案

2.5x-2y+5.5x
=（2.5+5.5）x-2y
=8x-2y
=8×0.5-2×0.9
=4-1.8
=2.2