So let's see we have some arbitrary point - let's define some arbitrary point x right over here then this would be the point (x,f(x)) and then we could take some x+h - so let's say that this right over here is the point x+h, so this point would be (x+h,f(x+h)), we can find the slope of the secant line, that goes between these two points, so that would be your change in your vertical, which would be f(x+h), f of x plus h, minus f(x), minus f(x), over the change in the horizontal, which would be x+h minus x, minus x; and these two x's cancel, so this would be the slope of this secant line, and if we wanna find the slope of the tangent line at x - the slope of the tangent line at x- we would just take the limit - the limit of this expression - as h approaches 0, as h approaches 0, this point moves toward x, and that slope of the secant line between these two is going to approximate the slope of the tangent at x
ลองดู เรามีจุดตามใจจุดหนึ่ง -- ลองกําหนดจุดตามใจ x ตรงนี้ แล้วนี่ก็คือจุด (x, f(x)) แล้วเราก็หา x+h -- สมมุติว่าเจ้านี่ตรงนี้ คือจุด x+h, ดังนั้นพิกัด