2017秋九年级数学上册 2.3 第1课时 用公式法求解一元二次方程教案1 （新版）北师大版.doc

1、 1 2 3 1 1 2 ( ) 3 b 2 4ac ( ) ax 2 bx c 0(a ax 2 bx c 0(a b 2 4ac x1 b b 2 4ac 2a x2 b b 2 4ac 2a . 3x 2 8 7x _ a _ b _ c _ _ _ 3x 2 7x 8 0. a 3 b 7 c 8 b 2 4ac ( 7) 2 8) 145 0 x 145 6 . 3x 2 7x 8 0 3 7 8 145 6 . ax 2 bx c 0(a 0) a b c a b c (1) 3x 2 5x 2 0; (2)2x 2 3x 3 0 (3)x 2 2x 1 0. a b c b 2

2、4ac (1) 3x 2 5x 2 0 3x 2 5x 2 0. a 3 b 5 c 2 b 2 4ac 5 2 2) 49 0 x 49 6 x1 1 3 x2 2 a 2 b 3 c 3 b 2 4ac 3 2 9 24 15 0 a 1 b 2 c 1 b 2 4ac ( 2) 2 0 x 0 2 x1 x2 1. a b c b 2 4ac ( ) x 2 x 1 ( ) A B C D x 2 x 1 b 2 4ac 1 2 1) 5 0 B. 2 ax 2 bx c 0(a b 2 4ac 0 b 2 4ac 0 b 2 4ac 0 x kx 2 2x 1 0 k ( ) A k 1 B k 1 k C k0 0 2 2 k 1 0 k k 1 k B. b 2 4ac 0 A. a b c ABC m0 x c(x 2 m) b(x 2 m) 2 m ax 0 ABC a b c ABC (b c)x 2 2 m ax (c b)m 0. 2 m a) 2 4(b c)(c b)m 0 4m(a 2 b 2 c 2 ) 0. m a 2 b 2 c 2 0 a 2 b 2 c 2 . ABC .