Question

初三数学，在线等，急急急急急急急~~

y=（10+x)(500-10x)
y=-10x²+400x+5000
y=-10（x²-40x）+5000
=-10（x-20）²+9000
我想知道最后一步中的9000是怎么得来的
谢谢啊，亲们

Answer 1

配方X方-40X要加400才能配方
400* （-10）=4000
因为里面多加了400
所以要减去400才能保证数不变
-400乘以10=4000
4000+5000=9000
所以 最后一步中的9000

Answer 2

y=(10+x)(500-10x)
y=5000-100x+500x-10x²
y=5000+400x-10x²
y=-10x²+400x+5000
把-10提出来
y=-10(x²-40x-500)
y=-10(x²-40x+20²-20²-500)
y=-10[(x-20)²-900]
y=-10(x-20)²+9000

Answer 3

y=-10x²+400x+5000
=-10x²+400x-4000+4000+5000
=-10(x²-40x+400)+9000
=-10(x-20)^2+9000

Answer 4

=-10（x²-40x）+5000
=-10 (x²-40x+400-400) +5000
=-10 [ (x-20)²-400 ]+5000
=-10 (x-20)²+9000