小組建設(shè)課-有理數(shù)的乘法

1、有理數(shù)的乘法自貢市解放路中學(xué)自貢市解放路中學(xué) 李燃李燃有理數(shù)的乘法法則有理數(shù)的乘法法則:兩數(shù)相乘兩數(shù)相乘,同號(hào)得同號(hào)得_,異號(hào)得異號(hào)得_,絕對(duì)值絕對(duì)值_.任何數(shù)同任何數(shù)同0相乘積為相乘積為01.1.閱讀教材閱讀教材, ,理解有理數(shù)的乘法法則的合理性理解有理數(shù)的乘法法則的合理性正正負(fù)負(fù)相乘相乘2.2.法則熟悉法則熟悉: :口答口答, ,說(shuō)出下列兩數(shù)積的符號(hào)說(shuō)出下列兩數(shù)積的符號(hào). .(1)5x(-3) (2)(-4)x1/2 (3)-1/7x(-9)(4)0.5x0.7 (5) -5x(-2) (6)- -2x2計(jì)算原則計(jì)算原則:先定符號(hào)先定符號(hào),再算結(jié)果再算結(jié)果正正正正負(fù)負(fù)負(fù)負(fù)負(fù)負(fù)負(fù)負(fù)例例1.計(jì)

2、算計(jì)算(1)(-4)x5 7x(-5)(2)(-5)x(-7) (-6)x(-9)(3)(-3/8)x(-8/3) (-3)x(-1/3)二、合作探二、合作探“究究”（互幫互助我努力）（互幫互助我努力）倒數(shù)定義倒數(shù)定義:乘積為乘積為1的兩個(gè)有理數(shù)互為倒數(shù)的兩個(gè)有理數(shù)互為倒數(shù),互為倒數(shù)的兩個(gè)數(shù)符號(hào)相同互為倒數(shù)的兩個(gè)數(shù)符號(hào)相同,0沒(méi)有倒數(shù)沒(méi)有倒數(shù)絕對(duì)值是本身的數(shù)是絕對(duì)值是本身的數(shù)是_.倒數(shù)等于本身的倒數(shù)等于本身的數(shù)是數(shù)是_.相反數(shù)等于本身的數(shù)是相反數(shù)等于本身的數(shù)是_.-20-35355411非負(fù)數(shù)非負(fù)數(shù)1,-10例2.計(jì)算(1)(-4)x5x(-0.25) 0.5x(-7)x(-4)(2)(-3/

3、5)x(-5/6)x(-2) (-85)x(-25)x(-4)小結(jié)：小結(jié)： 幾個(gè)不為零的因數(shù)像相乘:負(fù)數(shù)的個(gè)數(shù)為偶數(shù)個(gè)時(shí),積為正數(shù),負(fù)數(shù)的個(gè)數(shù)為奇數(shù)個(gè)時(shí),積為負(fù)數(shù),并把絕對(duì)值相乘簡(jiǎn)稱簡(jiǎn)稱:奇負(fù)偶正奇負(fù)偶正!三、精要三、精要“講講”解（聚精會(huì)神我認(rèn)真）解（聚精會(huì)神我認(rèn)真）1.計(jì)算(1)(-6)x(-7) (2)(-5)x12 (3)0.5x(-0.4) (4)-4.5x(-0.32)2.計(jì)算(1)1/2x(-4/7) (2)(-5/6)x(-3/10) (3)-4/15x5x(-0.3)x(-10/7)反思拓展反思拓展:1.有理數(shù)乘法法則有理數(shù)乘法法則:兩數(shù)相乘兩數(shù)相乘,同號(hào)得同號(hào)得_,異異號(hào)得

4、號(hào)得_,絕對(duì)值絕對(duì)值_.2.乘積為乘積為_(kāi)的兩個(gè)有理數(shù)互為倒數(shù)的兩個(gè)有理數(shù)互為倒數(shù),_沒(méi)有倒數(shù)沒(méi)有倒數(shù),_的倒數(shù)是本身的倒數(shù)是本身.3.幾個(gè)不為零因數(shù)相乘幾個(gè)不為零因數(shù)相乘:負(fù)數(shù)的個(gè)數(shù)為偶數(shù)負(fù)數(shù)的個(gè)數(shù)為偶數(shù)個(gè)時(shí)個(gè)時(shí),積為積為_(kāi)數(shù)數(shù),負(fù)數(shù)的個(gè)數(shù)為奇數(shù)個(gè)時(shí)負(fù)數(shù)的個(gè)數(shù)為奇數(shù)個(gè)時(shí),積為積為_(kāi)數(shù)數(shù).4.有一個(gè)因數(shù)是有一個(gè)因數(shù)是0時(shí)時(shí),積為積為_(kāi).正正負(fù)負(fù)相乘相乘101或或-1正正負(fù)負(fù)0四、學(xué)以致四、學(xué)以致“用用”（舉一反三我成功）（舉一反三我成功）1.計(jì)算:(1)(-16)x15= (2)(-9)x(-14)= (3)(-36)x(-1)=(4)100 x(-0.001)= (5)-4.8x(-1.25)= (6)-4.5x(-0.32)=2.填空(用”或”號(hào)連接):(1)如果a0,那么ab_0(2)如果a0,b0時(shí),那么a_2a(4)如果a0時(shí),那么a_2a五.(-2/3)x(-3/10)x(-20/