0
$\begingroup$

I need help with three homework questions.¨

First one:

$$\sqrt{3x^2-2x-15}=x+1$$ I don't know how to get the right answer. The answer is supposed to be 4. I get: $$ 3x^2-2x-15=(x+1)^2$$ $$ 3x^2-2x-15=x^2+2x+1$$ $$ 2x^2-4x-14=0$$ $$ x^2-2x-7=0$$

$$ x_1=1+\sqrt8$$ $$ x_2=1-\sqrt8$$

Second question is: $$ ln(x-7)-2ln(x-1)=-2ln5$$ where i come to this stage but don't know how to solve it $$\frac{x-7}{(x-1)^2}=\frac{1}{25}$$

the answer is supposed to be $x=11$ or $x=16$

Third question is $$9^x-4*3^x+3=0$$

I have come this far $$3^{2x}-4*3^x+3=0$$

answer is supposed to be $x=1$ or $x=0$

$\endgroup$
  • $\begingroup$ 1st question: when you went from $-15$ to $-14$ you should have gone to $-16$ instead. $\endgroup$ – vadim123 Oct 28 '15 at 22:46
  • $\begingroup$ Welcome to the site! In the future, please separate your questions into different posts. $\endgroup$ – Théophile Oct 28 '15 at 23:01
0
$\begingroup$

1) $-15 - 1 = -16$ not $-14$.

2) multiply by sides to get: $25(x -7) = (x - 1)^2$

3) $3^{2x} + 4*3^x + 3 = 0$. Let $y = 3^x$ so to get $y^2 + 4y + 3 = 0$. Easier?

$\endgroup$
  • $\begingroup$ I understand question 1) and 3) now thank you. on question 2 I dont't understand how that helps me? shall I change forms to $$25x-175=x^2-2x+1$$? I don't understand how that makes it easier. $\endgroup$ – Fredrik Nyberg Oct 28 '15 at 23:14
  • $\begingroup$ You just need to solve for x, don't you? $ 25(x−7)=(x−1)^2 \implies 25x−175=x^2−2x+1 \implies x^2 - 27x + 176 = 0 $ (if I didn't make any mistakes which I probably did.--- no, i didn't) $\endgroup$ – fleablood Oct 29 '15 at 0:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.