The substitution property of equality is a part of its definition; you can substitute anywhere.
The substitution property of equality is a part of its definition; you can substitute anywhere.
The choice is obvious to you. Not to the average American.
Similarly, 1/3 = 0.3333…
So 3 times 1/3 = 0.9999… but also 3/3 = 1
Another nice one:
Let x = 0.9999… (multiply both sides by 10)
10x = 9.99999… (substitute 0.9999… = x)
10x = 9 + x (subtract x from both sides)
9x = 9 (divide both sides by 9)
x = 1
North Korea: 316 downloads
Interesting that an Israeli newspaper provides a more balanced report than US outlets… how did that happen?
I’m pretty sure that would be ex uno, plura
For any
a
,b
,c
, ifa = b
andb = c
, thena = c
, right? The transitive property of equality.For any
a
,b
,x
, ifa = b
, thenx + a = x + b
. The substitution property.By combining both of these properties, for any
a
,b
,x
,y
, ifa = b
andy = b + x
, it follows thatb + x = a + x
andy = a + x
.In our example,
a
isx'
(notice the'
) andb
is0.999…
(by definition).y
is10x'
andx
is9
. Let’s fill in the values.If
x' = 0.9999…
(true by definition) and10x = 0.999… + 9
(true by algebraic manipulation), then0.999… + 9 = x' + 9
and10x' = x' + 9
.If you actually change any of the sides. Since, after substitution, the numeric value doesn’t change (literally the definition of equality), I don’t have to do anything – as I’m not rearranging. I’m merely presenting the same value in an equivalent manner. By contrast, when multiplying both sides by 10, since multiplication by 10 changes the concrete numeric value, I have to do it on both sides to maintain the equality relation (ditto for subtracting
x'
). But substitution never changes a numeric value – only rearranges what we already know.(Edit)
Take the following simple system of equations.
How would you solve it? Here’s how I would:
Here’s how Microsoft Math Solver would do it.