A cuboid with a square bottom is just a square after unfolding its side. The surface area of the cuboid is The surface area of a cuboid is () times that of a bottom area

The height is equal to the perimeter of the bottom
So it's four times as high as the bottom
Surface area = 2 × bottom area + 4 × 4 × bottom area = 18 × bottom area
So the surface area of a cuboid is (18) times that of a base

If we reduce the length of a rectangle by 15cm and increase its width by 6cm, we can get a square, and the areas of the two figures are exactly the same
What's the length and width of this rectangle?
To set up a system of linear equations of two variables, once!

Let the original length be a and the width be B, then A-15 = B + 6, and because the areas are equal, the increased area is equal to the reduced area. Then there is 15b = 6 (A-15), and the two forms are simultaneous, a = 25, B = 4

When the length of a rectangle is reduced by 15cm and the width is increased by 6cm, it becomes a square, and the areas of the two figures are equal

Let the length of a rectangle be x cm and the width be y cm
x-15=y+6
Infer: x = y + 21
According to the equal area, we can get
xy=(y+6)^2
The solution is y = 4
x=25
The area of the rectangle is 4 × 25 = 100 square centimeters

There is a rectangular piece of paper. Fold one corner of the paper as shown in the figure to find the area of the shadow in the figure. (unit: cm)

7 × 4-12 × (7-4) × 4 × 2 = 28-12 × 3 × 4 × 2 = 28-12 = 16 (cm2) a: the area of the shadow in the figure is 16cm2

A cuboid with a square bottom is just a square after unfolding its side. The surface area of the cuboid is The surface area of a cuboid is () times that of a bottom area