When you solve for the 2nd derivative, you are determining whether the function is concave up/down. If you calculated that the 2nd derivative is negative, the function is concave down, which means you have a relative/absolute maximum, given that the 1st derivative equals 0.
To understand why this is, think about the definition of the 2nd derivative. It is a measure of the rate of change of the gradient. At a maximum, the gradient starts positive, becomes 0 at the maximum itself and then becomes negative, so it is decreasing. If the gradient is going down, then its rate of change, the 2nd derivative, must be negative.
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The anti derivative of negative sine is cosine.
-cos(x)
It is negative one divided by 4 multiplied by x to the power of 1.5 -1/(4(x^1.5))
the second derivative at an inflectiion point is zero
- e^- X