23W+ U=- 2u2.,;2 ^ 1914-t a CO the sang tac va
giai dUdc nhien phitdng t r i n h hay va kho, thudng gaptrong cac k}' t h i hoc sinh gioi. D6 van dung dildc phitdng phap nay, t a thirdngbien ddi phiWng t i i n h da cho thanh phitdng t r i n h ham f {(ipix)),trong do / la ham ddn dicu. Tfr day[r]