P = 9 ( x+1)^2 + ( x-5 )^2 -2 ( x^2 +8 ) +21

a)

P = 9(x + 1)^2 + (x - 5)^2 - 2(x^2 + 8) + 21

P = 9(x^2 + 2x + 1) + (x^2 - 10x + 25) - 2(x^2 + 8) + 21

P = (9x^2 + 18x + 9) + (x^2 - 10x + 25) - (2x^2 + 16) + 21

P = 9x^2 + 18x + 9 + x^2 - 10x + 25 - 2x^2 - 16 + 21

P = (9x^2 + x^2 - 2x^2) + (18x - 10x) + (9 + 25 - 16 + 21)

P = 8x^2 + 8x + 39

b) Thay x = -0,4 vào P, ta có:

P = (-0,4)^2 + 8.(-0,4) + 39

P = 0,16 - 3,2 + 39

P= 35,96

@vietdorapan