幾題數~~~~~ 聽日要交~~~~~~~ (要steps)

11. 1/a = 2b + 1/c
c/a= 2bc + 1
c(1/a -2b) = 1
c=1/(1/a -2b)

7. 2(a-b)(c+2d) - (b-a)(3e-2c)
= (a-b)(2c+4d) + (a-b)(3e-2c)
= (a-b)[(2c+4d)+(3e-2c)]
= (a-b)(3e+4d)
8. ??
9. -6/r-3 - 2r/3-r
= -6/r-3 + 2r/r-3
= 2r-6/r-3
10. sub.s = -4 and a = 9
s = a+b / a-b
-4 = 9+b / 9-b
4b -36 = 9+b
3b = 45
b = 15
11. 1/a = 2b + 1/c
c/a= 2bc + 1
c(1/a -2b) = 1
c=1/(1/a -2b)
12. n = 50/2 = 25
S = 25(25+1)
= 650
13. 3(x-A)
= 3x - 3A
by comparing the coefficient
B=3
-3A = 4
A =- 4/3
14. (x-A)^2 = x^2 -2Ax +A^2
by comparing the coefficients
C = 1
-2A = -6
A=3
A^2 = B
3^2 = B
B=9 //

2008-01-14 22:48:47 補充：
if 8. is Simplify 4b/3b 3 / 6b / ab a4b/3b 3 / 6b / ab a= 4b(ab a) / 6b(3b 3)= 2a(b 1) / 3x3(b 1)= 2a/9 //

12. The sum of the first n positive even integers is given by S = n(n+1). Find the value of 2+4+6+...+50.
2(1+2+3...+25)(因式分解)
=650