How to tackle problems on Change of Subject Formula

Change of Subject Formula is all about rearranging a formula or relation to change the subject of that formula we will apply alot of rules, how signs change when they cross equality signs, but i intend solving examples on complex examples so watch the video below to learn the introduction and basics.


Now I will solve three questions, you will watch the rest in the videos listed in this post.

     Questions.

1. Make x the subject of formula in h=c+x/c-x
   Solution
h=c+x/c-x
If you look carefully you will find x both at the denominator and numerator, and whenever you see  a fraction like that you try to cross multiply.
h(c-x) =c+x   Expand the brackets
hc-hx=c+x  now we collect like terms,
hc-c=x+hx now lets factorise x out.
hc-c=x(1+h) now divide both sides by 1+h to make h alone
hc-c/1+h =x(1+h)/1+h
x=hc-c/1+h.

2. make c the subject of formula in x=-b±√b²-4ac/2a.
As I said you cross multiply when you what you are looking for is part of a fraction. cross multiplying we have :
x(2a)= -b±√b²-4ac  to isolate the square root add +b to both sides to eradicate - b at the right hand side.
2ax+b=±√b²-4ac ignore the ± sign, now to remove square root you need to square both sides.
(2ax+b)² = (√b²-4ac)²
(2ax+b)(2ax+b)=b²-4ac
4a²x²+4abx+b² = b² - 4ac.
To isolate 4ac subtract b² from both sides.
4a²x²+4abx+b²-b² = b² - 4ac -b², the b² will subtract itself completely from both sides, we now have:
4a²x²+4abx=-4ac, lets factorise 4a at our left hand side.
4a(ax²+bx) = - 4ac.

To make c alone divide both sides by -4a.

we have 4a(ax²+bx)/-4a = -4ac/-4a

c=-(ax²+bx) = - ax²-bx.

If you remember the quadratic general equation 

ax²+bx+c=0.

if you transfer ax² and bx to the other side, c will be alone which will still give 

c=-ax²-bx.

3. make u the subject of formula in 1/f=1/u + 1/v

first of all lets find the lcm of u and v, we will have uv. finding a common denominator we have 

1/f=v+u/uv. cross multiply 1(uv) = f(v+u).  Expand the bracket. 

uv=fv+fu.  collect like terms we have. 

uv-fu=fv.  factorise u out at the LHS. 

u(v-f) =fv.  divide both sides by v-f, we have 

    u=fv/v-f.

To make t the subject of formula in the compound interest formula and more watch the video below, if you are confused with my post you can also watch the video.


 If you have any formula that you can't rearrange drop a comment below. Thanks for reading my post don't forget to share this post with your friends