plz help me to do the question.If you do not know the answer,plz answer no!ThX!
1. Find 1x2+2x3+3x4+........+10x11.
2.The dimensions of the floor of a room are amxbm.If the floor is covered with carpet which costs $x per m²,the cost of covering is:
A.$x(a+b)
B.$(xa+b)
C.$xab
D.$2xab
math difficult question/F.1
2007-07-23 3:16 am
回答 (4)
2007-07-23 3:24 am
✔ 最佳答案
1x2+2x3+3x4+4x5+5x6+6x7+7x8+8x9+9x10+10x11= 2x[4]+4x[8]+6x[12]+8x[16]+10x[20]
= 8+32+72+128+200
= 40+ 200+ 200
= 440
2
xab
C
The total m² covered is ab
1 m² cost x
ab² cost xab
p.s. 第一條應該唔駛咁計, 我搵搵睇睇有無得prove/ pattern
2007-07-23 10:40 pm
Sum(n(n+1))
=Sum(n^2)+Sum(n)
=1/6(n)(n+1)(2n+1)+1/2(n)(n+1)
=1/3(n)(n+1)(n+2)
but n=10, so:
1/3(10)(11)(12)
=(10)(11)(4)
=440
=Sum(n^2)+Sum(n)
=1/6(n)(n+1)(2n+1)+1/2(n)(n+1)
=1/3(n)(n+1)(n+2)
but n=10, so:
1/3(10)(11)(12)
=(10)(11)(4)
=440
2007-07-23 3:28 am
Q1
1x2 + 2x3 + 3x4 + 4x5 + 5x6 + 6x7 + 7x8 + 8x9 + 9x10 + 10x11
= 2+6+12+20+30+42+56+72+90+110
=70 + 170 +90 +110
= 440
Q2
C:$xab
1x2 + 2x3 + 3x4 + 4x5 + 5x6 + 6x7 + 7x8 + 8x9 + 9x10 + 10x11
= 2+6+12+20+30+42+56+72+90+110
=70 + 170 +90 +110
= 440
Q2
C:$xab
2007-07-23 3:24 am
1. n(n+1)=n^2 +n
So, 1x2+2x3+3x4+........+10x11
= (1^2+2^2+......10^2) + (1+2+....+10)
=385+55
=440
2. area= ab m²
so, cost = $ xab, so C is the answer
So, 1x2+2x3+3x4+........+10x11
= (1^2+2^2+......10^2) + (1+2+....+10)
=385+55
=440
2. area= ab m²
so, cost = $ xab, so C is the answer
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