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