有幾條maths想問..

1. x(x+2)-(x+2)
=x(x+2)-1*(x+2)
=(x+2)(x-1)

2. (s+1)t-1-s
=(s+1)t-(1+s)
=(s+1)(t-1)

3. 3(a-b)(3r-t)+9(b-a)(2t-5s)
=3(a-b)(3r-t)-9(a-b)(2t-5s)
=3(a-b)(3r-t)-3(a-b)*3(2t-5s)
=3(a-b)[(3r-t)-3(2t-5s)]
=3(a-b)(3r+15s+7t)

1) x(x+2)-(x+2)
=x二次方+2x-x+2

2) (s+1)t-1-s
=st+t-1-s

3) 3(a-b)(3r-t)+9(b-a)
=3(3ra-at-3rb+bt)+9b-9a
=9ra-3at-9rb+3bt+9b-9a