The current through a resistor, in amperes, is I = 4.85 - 0.001t^2

The resistance, in ohms, is R = 15 +0.11t ( t is in seconds).

The voltage, in volts, is the product of the current and the resistance.

How long will it take for voltage to reach 77V, to the nearest second?

I found V(t) to be V(t) = IR

Therefore, I just multiplied and brought everything out of the bracket to get:

v(t) = -0.00011t^3 - 0.015t^2 + 0.5335t + 72.75

which is what is says in the Answers.

I subbed V(t) = 77, but now I don't know how to isolate for t. We aren't too familiar with factoring cubic equations, so I'd like some help.

Thanks. : )

Also, how would you sketch this function?

For example, if this was a quadratic equation, I could complete the square to find where the vertex and zeroes are and etc. to sketch it.

Is there a method similar to completing the square for cubic equations, though?