# Thread: Expanding, factorising and transposing questions

1. ## Expanding, factorising and transposing questions

1) expand and simplify:

(x-1/x)2

2) factorise:
x^2+6x+8

3) transpose to make t the subject:
v=200e^(-Rt/L)

2. Originally Posted by clayson
1) expand and simplify:

(x-1/x)2

2) factorise:
x^2+6x+8

3) transpose to make t the subject:
v=200e^(-Rt/L)

1)$\displaystyle \frac{2x-2}{x}$

2) (x+2)(x+4)

3) log? or ln?

3. yep base e

3) log?[/quote]

4. Originally Posted by clayson
yep base e

3) log?
Got it.

$\displaystyle t=\frac{ln\frac{v}{200}L}{-R}$

:P

5. The voltage drop, v volts, across and inductor L henrys at time t seconds is given by: v=200e^(-Rt/L)

R=150 ohms
L=12.5*10^-3 H

What time does it take for the voltage to reach 85 volts>

could you go through it step by step if possible and also for the (x-1/x)^2 question.

Thanks BabyMilo for the quick reply.

6. Originally Posted by clayson
The voltage drop, v volts, across and inductor L henrys at time t seconds is given by: v=200e^(-Rt/L)

R=150 ohms
L=12.5*10^-3 H

What time does it take for the voltage to reach 85 volts>

could you go through it step by step if possible and also for the (x-1/x)^2 question.

Thanks BabyMilo for the quick reply.
for a) which one is it? (x-1/x)^2 or (x-1/x)2?

they are quite different.!

7. Originally Posted by clayson
The voltage drop, v volts, across and inductor L henrys at time t seconds is given by: v=200e^(-Rt/L)

R=150 ohms
L=12.5*10^-3 H

What time does it take for the voltage to reach 85 volts>

could you go through it step by step if possible and also for the (x-1/x)^2 question.

Thanks BabyMilo for the quick reply.
$\displaystyle v = 200e^{\frac{-Rt}{L}}$

$\displaystyle \frac{v}{e^{\frac{-Rt}{L}}} = 200$

$\displaystyle e^{\frac{-Rt}{L}} = \frac{v}{200}$

$\displaystyle \frac{-Rt}{L} = ln(\frac{v}{200})$

$\displaystyle -Rt = ln(\frac{v}{200})*L$

$\displaystyle t = \frac{ln(\frac{v}{200})*L}{-R}$

sub in the numbers.

$\displaystyle t = \frac{ln(\frac{85}{200})*12.5*10^{-3}}{-150}$

t = 71.31×10^−6s

8. (x-1/x)^2 sorry for the mistake.

9. Originally Posted by clayson
(x-1/x)^2 sorry for the mistake.
$\displaystyle (\frac{x-1}{x})^2$

$\displaystyle \frac{(x-1)^2}{x^2}$

$\displaystyle \frac{1}{x^2}-\frac{2}{x}+1$