Question

在等式y ax的平方＋bx＋c当x=－1时,y=4;当x=1时,y=8;当x=2与y=25时,则当x＝3时的值是多少

Answer 1

y = ax^2 + bx + c

当x = -1,y = 4时,a - b + c = 4 (1)

当x = 1,y = 8时,a + b + c = 8 (2)

当x = 2 ,y = 25时,4a + 2b + c = 25 (3)

联立（1）（2）（3）组成的不等式组得：a = 5,b = 2 ,c = 1,

所以 y = 5x^2 + 2x + 1

当 x = 3时,y = 5×3^2 + 2×3 + 1 = 52