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The process of 3-4 questions on page 40 of the third grade mathematics book published by Beijing Normal University.
3. Solution: ∵ If the length and width are both x meters, then the height is (x+0.5) meters.

∴( 1) surface area is

S=[x? +x(x+0.5)+x(x+0.5)]×2

=(x? +x? +0.5x+x? +0.5 times) ×2

=(3x? +x)×2

=6x? +2 times

②y = 5S

=5(6x? +2 times)

=30x? + 10x

4. solution: y=(x-20)t

=(x-20)(-3x+70)

=-3x? + 130x- 1400