6,666,666×666,666 /1+2+3+4+5+6+5+4+3+2+1 − 777,777×777,777/1+2+3+4+5+6+7+6+5+4 +3+2+1

111111 1111110 is the simple expression.

Step-by-step explanation:

The expression to be solved:

First of all, let us solve the first term:

Now, the right term:

So, the expression to be solved becomes:

answer: a

step-by-step explanation:

we have three similar triangles, because each has a right angle and shares an angle.   let's write the angles in order: opposite to short leg, long leg, hypotenuse.

cab similar to bad similar to cbd

or as ratios,

ca: ab: cb = ba: ad: bd = cb: bd: cd

we also know

ac = ad + cd

(ad+cd): ab: cb = ba: ad: bd = cb: bd: cd

(ad+cd)/cb=cb/cd

we have cb=6, ad=5 and seek x=cd.

we reject the negative root and conclude x=4

answer: 4