Answer:

Step-by-step explanation:
The Pythagorean Theorem is used to find a missing side length in a right triangle, and the formula
is used to find the missing side, assuming a and b are legs, while c is the hypotenuse.
You can write a system of equations, I'm pretty sure.
the first equation would be
10a+3f+.5c=100
and
a+f+c=100
For the first equation, its the price of each ticket that adds up to 100 tickets
For the second equation, its the amount of people that adds up to 100 people.
I'm pretty sure this is the route to go but I haven't solved it for myself (yet) I'll probably comment the answer if you need me to
Use the definition of tangent
or inv tangent (arctan or atan)
tan(theta) = opposite side/ adjacent side
the vertical rise would be the opposite side, the horozontal distance would be the adjacent side
let rise = y
let distance= x
x=2y
so tan(theta) = y/2y
tan(theta) = 1/2
theta = atan(1/2)
grab a handy dandy calculator, unless you know this conversion off the top of your head
3 x 12 would equal 36, +1 would make it 37. Since there is no space in between 2 and the (, you multiply it. 37 x 3 = 111
So, Jeff should order 111 books

![\bf \stackrel{\textit{multiplying both sides by LCD of 3}}{3(y+5)=3\left[ \cfrac{5}{3}(x-3) \right]}\implies 3y+15=5(x-3) \\\\\\ 3y+15=5x-15\implies -5x+3y=-30\implies \stackrel{\textit{multiplying by -1}}{5x-3y=30}](https://tex.z-dn.net/?f=%5Cbf%20%5Cstackrel%7B%5Ctextit%7Bmultiplying%20both%20sides%20by%20LCD%20of%203%7D%7D%7B3%28y%2B5%29%3D3%5Cleft%5B%20%5Ccfrac%7B5%7D%7B3%7D%28x-3%29%20%5Cright%5D%7D%5Cimplies%203y%2B15%3D5%28x-3%29%0A%5C%5C%5C%5C%5C%5C%0A3y%2B15%3D5x-15%5Cimplies%20-5x%2B3y%3D-30%5Cimplies%20%5Cstackrel%7B%5Ctextit%7Bmultiplying%20by%20-1%7D%7D%7B5x-3y%3D30%7D)
bearing in mind the standard form uses all integers, and the x-variable cannot have a negative coefficient.