Posted by **Andres** on Tuesday, October 11, 2011 at 8:15am.

Suppose Y1 is a function of x which dy1/dx=3y1. Suppose y2 is a function of x which dy2/dx=8x+5. If the graphs of y1 and y2 have the same y-intercept and they intersect at x=2, then determine the value of the y-intercept.

- Calc -
**Reiny**, Tuesday, October 11, 2011 at 8:44am
if dy1/dx = 3y1

then y1 = ae^(3x) , where a is a constant, a≠ 0

(check: dy1/dx = 3a(e^(3x)) = 3y1 )

if dy2/dx = 8x + 5

then y2 = 4x^2 + 5x + c

the y-intercept of y1 is (0,a), but that is also the y-intercept of y2

so in y2:

a = 0 +0 + c

a = c

when x=2

y1 = a(e^6) , y2 = 16+10+c = c + 26

but a=c, so

a(e^6) = a+26

a(e^6) - a = 26

a(e^6 - 1) = 26

a = 26/(e^6 - 1) , which would be the y-intercept

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