diện tích HCN đó là:28:25x100=112(cm2)

tick cho mk nha

A)\(\dfrac{a}{b}=\dfrac{c}{d}=\dfrac{a}{c}=\dfrac{b}{d}\)

áp dụng tính chất dãy tỉ số bằng nhau ta có

\(\dfrac{a}{c}=\dfrac{b}{d}=\dfrac{a-b}{c-d}\)=\(\dfrac{a}{a-b}=\dfrac{c}{c-d}\) (đpcm)

a=7

bài 1:

a. 4n-3 ⋮n-2

vì (n-2)⋮(n-2)

=> 4(n-2)⋮(n-2)

=> 4n-8⋮(n-2)

=> (4n-3)-(4n-8)⋮(n-2)

=> (4n-3-4n+8)⋮(n-2)

=> 5⋮(n-2)

=> n-2∈Ư(5)=\(\left\{\pm1;\pm5\right\}\)

ta có bảng sau

vậy x∈{-3;1;3;7}

x³-7x-6

x³+0x²-7x-6

=x³+x²-x²-x-6x-6

= (x³+x²)-(x²+x)-(6x+6)

=x²(x+1)-x(x+1)-6(x+1)

= (x+1)(x²-x-6)

= (x+1)(x²+2x-3x-6)

= (x+1) [x(x+2)-3(x+2)]

=(x+1) (x-3)(x+2)

(a²-a+2012)(a²-a+2014)-3

đặt a²-a+2012=x => a²-a+2014=x+2

ta có x(x+2)-3

= x²+2x-3

=x²+3x-1x-3x

=x(x+3)-1(x+3)

=(x-1)(x+3)

thay x= a²-a+2012

=> (a²-a+2012-1)(a²-a+2012+3)

=(a²-a+2011)(a²-a+2015)

2. a²( b - c ) + b² ( c - a ) + c² ( a - b)

= a²b-a²c+b²c-b²a+c²(a-b)

=(a²b-b²a)-(a²c-b²c)+c²(a-b)

= ab(a-b)-c(a²-b²)+c²(a-b)

=ab(a-b)-c(a-b)(a+b)+c²(a-b)

=(a-b)(ab-c(a+b)+c²)

= (a-b)[ab-ca-cb+c²]

= (a-b)[(ab-ca)-(cb-c²)]

= (a-b)[ a(b-c)-c(b-c)]

= (a-b)(b-c)(a-c)

2. a²( b - c ) + b² ( c - a ) + c² ( a - b )

= (a²b-a²c+b²c-b²a)+c²(a-b)

= (a²b-b²a)-(a²c-b²c)-c2(a-b)

= ab(a-b)-c(a2-b2)-c2(a-b)

= ab(a-b)-c(a-b)(a+b)-c²(a-b)

= (a-b)(ab-c(a+b)-c²)

= (a-b)(ab-ca-cb-c²)

=(a-b)[(ab-ac)+(-cb-c²)]

= (a-b)[a(b-c)-c(b-c)]

= (a-b)(b-c)(a-c)