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# s08week06 - Week 6,7: 2.5 Wronskian 3.1 Intro, Slope...

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Week 6,7: 2.5 Wronskian 3.1 Intro, Slope Fields, verify solution 3.2 Separable DE 3.3 Exact DE 3.4 Linear Equations 3.6 mixing/cooling ————– Problem: Verify that the function y = c 1 x is a solution of y ± = y 2 x Solution: Compute y ± and check. y ± = c 1 ( 1 2 ) x - 1 2 . y 2 x = c 1 x 2 x = c 1 ( 1 2 ) x ( x ) 2 = c 1 ( 1 2 ) 1 x = y ± . —————–

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2 Problem: Determine all values r so y = e rx is a solution to y ±± - 4 y ± + 3 y = 0 . Main Step: to solve linear DE dy dx + p ( x ) y = q ( x ) multiply by the integral factor f = e R p dx and use d dx ( fy ) = f ( y ± + py ) on the left to get d dx ( fy ) = fq. Problem: Solve dy dx + 2 x (1 - x 2 ) y = 4 x, - 1 x 1 . Solution: Find integral factor, inside integral ﬁrst: Z 2 x (1 - x 2 ) dx = - ln(1 - x 2 ) = ln ( (1 - x 2 ) - 1 ) (simplify!). ————– So e R 2 x ( 1 - x 2 ) dx = e ln ( (1 - x 2 ) - 1 ) = ( 1 - x 2 ) - 1 = 1 1 - x 2 = f.
3 Multiply by f = 1 1 - x 2 and use Main Property: 1 1 - x 2 ± dy dx + 2 x (1 - x 2 ) y = 4

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## This note was uploaded on 02/29/2008 for the course MATH 205 taught by Professor Zhang during the Spring '08 term at Lehigh University .

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s08week06 - Week 6,7: 2.5 Wronskian 3.1 Intro, Slope...

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