Question

Answer 1

20.

$A=(x+5)^2-(x-5)^2+(x+2)(x-2)-x^2+21$$=(x^2+10x+25)-(x^2-10x+25)+(x^2-4)-x^2+21$$=20x+17$

$\Rightarrow a=20,\ b=17$

$M=4a-5b$$=4\cdot20-5\cdot17$$=80-85$$=-5$

Vậy $M=-5$

21.

$A=(x+5)^2+(x-5)^2-2x^2+(x+3)(x-3)-x^2+59$$=(x^2+10x+25)+(x^2-10x+25)-2x^2+(x^2-9)-x^2+59$$=50-9+59$$=100$

$A=100$

Answer 2

`20.`

`A = (x + 5)^2 - (x - 5)^2 + (x + 2)(x - 2) - x^2 + 21`

`= (x^2 + 10x + 25) - (x^2 - 10x + 25) + (x^2 - 4) - x^2 + 21`

`= x^2 + 10x + 25 - x^2 + 10x - 25 + x^2 - 4 - x^2 + 21`

`= (x^2 - x^2 + x^2 - x^2) + (10x + 10x) + (25 - 25 - 4 + 21)`

`= 20x + 17`

`-> {(a = 20),( b = 17):}`

`M = 4a - 5b = 4 . 20 - 5 . 17= -5`

`21.`

`A = (x + 5)^2 + (x - 5)^2 - 2x^2 + (x + 3)(x - 3) - x^2 + 59

`= (x^2 + 10x + 25) + (x^2 - 10x + 25) - 2x^2 + (x^2 - 9) - x^2 + 59`

`= x^2 + 10x + 25 + x^2 - 10x + 25 - 2x^2 + x^2 - 9 - x^2 + 59`

`= (x^2 + x^2 - 2x^2 + x^2 - x^2) + (10x - 10x) + (25 + 25 - 9 + 59)`

`= 100`