2. จงหาผลบวก
1) -12.3 + 5.17 + (-3.24)
วิธีทำ
-12.3 + 5.17 + (-3.24)
= -12.3 + (-3.24) + 5.17
= [-12.3 + (-3.24)] + 5.17
= [-(12.3 + 3.24)] + 5.17
= (-15.54) + 5.17
= -(15.54 – 5.17)
= -10.37
= [-(12.3 + 3.24)] + 5.17
= (-15.54) + 5.17
= -(15.54 – 5.17)
= -10.37
ตอบ -10.37
2) -51.09 + (-8.2) + (-0.103)
วิธีทำ
-51.09 + (-8.2) + (-0.103)
= -(51.09 + 8.2 + 0.103)
= -59.393
ตอบ -59.393
3) -29.81 + 2.2 + 29.81
วิธีทำ
-29.81 + 2.2 + 29.81
= -29.81 + 29.81 + 2.2
= (-29.81 + 29.81) + 2.2
= 0 + 2.2
= 2.2
= 0 + 2.2
= 2.2
ตอบ 2.2
4) -10.45 + (-32.01) + 20.45
วิธีทำ
-10.45 + (-32.01) + 20.45
= -10.45 + 20.45 + (-32.01)
= (-10.45 + 20.45) + (-32.01)
= (20.45 – 10.45) + (-32.01)
= 10 + (-32.01)
= -(32.01 – 10)
= -22.01
= (20.45 – 10.45) + (-32.01)
= 10 + (-32.01)
= -(32.01 – 10)
= -22.01
ตอบ -22.01
5) 43.09 + (-0.602) + 7.91 + (-50.398)
วิธีทำ
43.09 + (-0.602) + 7.91 + (-50.398)
= 43.09 + 7.91 + (-0.602) + (-50.398)
= (43.09 + 7.91) + [(-0.602) + (-50.398)]
= 51 + [-(0.602 + 50.398)]
= 51 + (-51)
= 0
= 51 + [-(0.602 + 50.398)]
= 51 + (-51)
= 0
ตอบ 0
6) -12.03 + 11.54 + (-20.07) + 20.46
วิธีทำ
-12.03 + 11.54 + (-20.07) + 20.46
= -12.03 + (-20.07) + 11.54 + 20.46
= [-12.03 + (-20.07)] + (11.54 + 20.46)
= [-(12.03 + 20.07)] + 32
= (-32.1) + 32
= -(32.1 – 32)
= -0.1
= [-(12.03 + 20.07)] + 32
= (-32.1) + 32
= -(32.1 – 32)
= -0.1
ตอบ -0.1