中3 math 百分法

設原來的長方形的長是x,闊是y
新長方形面積
=x(1+18%)y(1-30%)
=0.826xy
長方形面積的變化百分比
=(0.826xy-xy)/xy*100%
= -17.4%
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其實x和y最後都會被約簡走,
所以可以簡單化為:
長方形面積的變化百分比
=[(1+18%)(1-30%)-1]*100%
= -17.4%

Let x and y be the length and width respectively.

Original area = xy

New length = 1.18x
New width = 0.7y

Hence, new area = 0.826xy

Thus, the 長方形面積的變化百分比 is 減少17.4%.

let original length is x, width is y
original area = xy

then, lenght increase 18% => 118%x
width decrease 30% => 70%y
the new area is now = 118% * 70% xy = 83% xy

so, the % of area change = 83% - 100% = -17%

設長方形長的原值是l, 闊的原值是w
長方形的原本面積=lw

長方形的新面積= l(1+18%) x w(1-30%)
=1.18l x 0.7w
=0.826lw

變化百分比 = (0.826lw-lw) / lw x 100%
=-17.4%

所以面積減少17.4%