英文數學方解題 (英文數學專家請進)

1)
Let the second side of the triangle be x cm

The first side of a triangle is 7cm shorter than twice the length of the second side
=> the length of the first side=2x-7

The third side is 4cm longer than the first side
=> the length of the third side=(2x-7)+4=2x-3

Perimeter is 80 cm
=> (2x-7)+x+(2x-3)=80
=> 5x=90
=> x=18

Hence, the three sides are 18,29,33

2)
Let the width be x m

The length of a rectangular field is 18 m longer than the width
=> the length=x+18

The total length of all the fencing is 216cm
=> 2(x+18)+3x=216
=> 5x=180
=> x=36

Thus, the length is 54m and the width is 36m

3)
Let the width be x m

The length of a rectangle is 7 m less than twice the width
=> the length=2x-7

When the length is decreased by 1m and the width is decreased by 4m, the perimeter equals 66m
=> 2[(2x-7)-1]+2(x-4)=66
=> 2(2x-8)+2(x-4)=66
=> 2x-8+x-4=33
=> 3x=45
=> x=15

Hence, the length is 23m and the width is 15m

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