Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Topic: Name this formula
Replies: 1   Last Post: May 28, 2013 2:24 PM

 Messages: [ Previous | Next ]
 Michael Posts: 2 From: sundsvall Registered: 5/28/13
Name this formula
Posted: May 28, 2013 11:43 AM
 Math problem.odg (8.7 K)

Today our teacher showed us a way to solve problems like this:
Theres a triangle with the base of 20cm and a line horizontal in the triangle that's 15cm. The line about the horzontal line is 8cm and the other one is X. Find x

Trying to explain this as good as i can but basicly its a triangle ABCDEF clockwise and theres a horisontal line between B and E (10 cm).
Then B->C=8cm and A->B=X
Finally the base (E->A) was 15cm.

(Hope most people get it)

He showed a way that almost took him up to 20 minutes to explain but i was thinking about this

15-10=5
10/5=2
That means 8 is 2 times bigger then X
so 8/2=4=x

I also found it work if we only knew the bottom one by changing it a little like this.
15-10=5
10/5=2
2x8=X=16 (simply changing the "divide by 2" into "multiply by 2")

But he said that I couldent use it unless i could prove it .
So is there allready a formula like this one or any way to prove it works? If it don´t I would like an example and reasoning why it dont work.

Date Subject Author
5/28/13 Michael
5/28/13 Steve B