急!!!數學證明題兩題

1.證明二次函數 f(x) =ax^2+bx+c (a < 0)在區間(負無窮,-b/2a]上是增函數
f(x) = ax^2 + bx + c
dy/dx = 2ax + b
when x < -b/2a
2ax > -b
2ax + b > 0
dy/dx > 0
so, when x < -b/2a, y = f(x) 是增函數

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2.證明函數 f(x) = x^3 + x (x属於R)在(負無窮,正無窮)上是增函數
dy/dx = 3x^2 + 1 > 0
y = f(x) 是增函數