y=2^x, y=3^x
the slope of the tangent line at x < 1, x > 1
the exponential function
y = e^x -> slope of the tange line is exactly 1.
e = lim n->∞(1 + 1/N)^N = 2.7
e^π = (1+ 3.1416/10,000)^10,000
(1 + x/4)^4 = 1 + x + 3/8x^2 + 1/16x^3 + 1/256x^4
e^x = 1 + x + x^2/2 + x^3/6 + x^4/24 + …
e^x+h = e^x * e^h
dy(x)/dx = -3y(x)
y(0) = 5
