Arithmetic sequence(10 points)

1)a)

let BC=x , then AB=x-d and CD=x+d ,

x+d+x+x-d=24

then x=8 // so BC=8.

b)

the Area of P1, P2 ,P3=105

(1/2)[(8+d)^2+(8-d)^2+8^2]=105
3*64+2d^2=210
d=3//

so the different between AB and BC=3//

c)

the length of JK=5+(10-1)3=32..

then the Area of P10=32^2/2=512//



2008-10-15 20:11:41 補充：
a)
1st=1*4
2nd=2*6
3rd= 3*8=24//
4th =4*10=40//

so the general term : n [2(n+1)]=2n(n+1)//