How do you find 1+1=?
sʇuᴉod ǝǝɹɟ

You are thinking of the Principia Mathematica, written by Alfred North Whitehead and Bertrand Russell. Here is a relevant excerpt (image below)

As you can see, it ends with "From this proposition it will follow, when arithmetical addition has been defined, that 1+1=2." The theorem above, ∗54⋅43, is already a couple of hundred pages into the book (Wikipedia says 370 or so); the later theorem alluded to, that 1+1=2, appears in section ∗102, considerably farther on.

I wrote a blog article a few years ago that discusses this in some detail. You may want to skip the stuff at the beginning about the historical context of Principia Mathematica. But the main point of the article is to explain the theorem above.

The article explains the idiosyncratic and mostly obsolete notation that Principia Mathematica uses, and how the proof works. The theorem here is essentially that if α and β are disjoint sets with exactly one element each, then their union has exactly two elements. This is established based on very slightly simpler theorems, for example that if α is the set that contains x and nothing else, and β is the set that contains y and nothing else, then α∪β contains two elements if and only if x≠y.

The main reason that it takes so long to get to 1+1=2 is that Principia Mathematica starts from almost nothing, and works its way up in very tiny, incremental steps. The work of G. Peano shows that it's not hard to produce a useful set of axioms that can prove 1+1=2 much more easily than Whitehead and Russell do.

It would not be hard to copy-and-paste the relevant parts of the blog article here, but I am not sure if that is appropriate se.math etiquette; I invite comments on this matter.

step-by-step explanation:

do you have a copy of the table?

c.   x=2.4cm

step-by-step explanation:

similar triangles are triangles which have the same shape but not the same size. this means their angle measures are equal but their side lengths are not instead they are proportional. to solve, we will set up a proportion and solve.

a proportion is two equal ratios set equal to each other. we form the ratios by finding corresponding sides (sides which match to each other on both triangles by their position). out ratios will be one side of the small triangle over the corresponding side on the big triangle as shown below:

.

to solve the proportion, we'll cross multiply by multiplying numerator with denominator across the equal sign.