Find f if f′(x)=20x^3+6x+5 and f(1)=−5.

What is f(x)?

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I get how top take the integral of 20x^3+6x+5 (=5x^4+3x^2+5x+C) but I dont understand where f(1)=-5 comes in. Can somebody help me out?

Question

Find f if f′(x)=20x^3+6x+5 and f(1)=−5.

What is f(x)?

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I get how top take the integral of 20x^3+6x+5 (=5x^4+3x^2+5x+C) but I dont understand where f(1)=-5 comes in. Can somebody help me out?

misty1212 · Answer

that is how you find C

anonymous · Answer

Would i plug 1 into x? or -5 into x?

misty1212 · Answer

$$f(1)=-5=5+3+5+C$$ solve for C

misty1212 · Answer

it is $f(1)=-5$ put in $1$ for $x$ set it equal to $-5$ and solve for $C$

anonymous · Answer

Ohhhhh so you are plugging f(1) into 5x^4+3x^2+5x+C?

misty1212 · Answer

yes

misty1212 · Answer

on account of it says $f(1)=-5$

anonymous · Answer

Ok thank you thats make much more sense than what I was trying to do. I was pluging it into  f'(x) and using what I got to solve f(x)

misty1212 · Answer

you are welcome, hope it is clear