Find the slope intercept form of the line whose slope is 4 and that passes through the point (-7,8) the equation of the line is?

- anonymous

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- MrNood

you know that the slope - intercept equation looks like this:
y=mx+b
OK?

- anonymous

Yes...

- anonymous

That's as far as I understand... I don't know how to get the answer.

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- MrNood

ok
well the 'm' in that equation is the slope
the question tells oyu the slope
so can you start to write the equation - putting th ecorrect value for m (but leave 'b' for now...)

- anonymous

Y=4x

- MrNood

y = mx +b
m = 4
so write the equation with the value for m in it

- MrNood

(you need the "+b" on the end )

- anonymous

Ok. How do you get b?

- anonymous

Oh, y=4x+b

- MrNood

at the moment I just wanted you to write the equation like this
y = 4x+b
we still need to find b
OK? - we are doing this in 2 steps - you have done one of them so far
Do oyu understand why the equation starts to be
y=4x+b ?

- anonymous

Yes..

- MrNood

OK good
So now the second step is to use the second bit of information from the question
that is the point (-7,8) lise on the line
this means that the equation is true when
x=-7, and y = 8
so write the equation
y = 4x+b
with the values x= -7 and y = 8 in it (leave b as it is for now)

- anonymous

Okay..

- MrNood

so write the equation
y = 4x+b
x= -7
y = 8
in it (leave b as it is for now)

- anonymous

Y=4x+b y =8x=-7 this doesn't make sense to me..

- MrNood

wherever there is y in the equation write 8
wherever there is x write -7 (BUT remember that 4x means 4 times x)

- anonymous

8=4-7+b yes?

- anonymous

B=5

- MrNood

yes - but to be clear about 4 times x it is clearer if oyu write
8 = 4(-7) +b

- anonymous

Y=4(-7)+5. Yes?

- MrNood

yes
so that can be rewritten as
8= -28 +b
so what is b?

- anonymous

Okay.. So I don't need it to equal 5?

- anonymous

B is 5 no??

- MrNood

no it's not , please don't rush - I am trying to guide you to work out the answer

- anonymous

Okay I see..

- anonymous

Sorry..

- MrNood

To review what we did..
we KNOW the equation will be in th eform
y = mx + b
and we know that m is th eslope
and we know the slope is 4
so we know th eequaiton is
y= 4x+b
BUT we also know that the point (-7,8) is on the line (which means the equationis true when it passes through that point)
SO we know that when x = -7 then y = 8
so back to the equation
we put y - 8 and x = -7
so
8=4×(−7)+b
this is the same as
8 = -28 + b
so you can work out b

- anonymous

Yes... B =36

- MrNood

well done
so now write the whole equation
y=mx+b
with the values for m and b that you calculated

- anonymous

Y=4x+36

- anonymous

Correct ?

- MrNood

correct
well done
this is the principle of all these questions -
once you know the slope, you can use 1 point on the line to find b

- anonymous

Thank you sir

- anonymous

So, I attempted my next one and the slope is 7 the point is (-4,11). I came up with y=7x+35 but I was wrong it is 39 and I don't know where I went wrong??

- MrNood

ok you made a good attempt
starting with
y = 7x + b
then we put in the point values
11= 7 (-4) +b
11 = -28 +b
you can see that 39 is the correct value

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