数学题速求18、19!要过程!
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数学题速求18、19!要过程!
![数学题速求18、19!要过程!](/uploads/image/z/16718718-30-8.jpg?t=%E6%95%B0%E5%AD%A6%E9%A2%98%E9%80%9F%E6%B1%8218%E3%80%8119%21%E8%A6%81%E8%BF%87%E7%A8%8B%21)
18,
= (4+0.4)+(-0.1)+(8+1/3)-(11+2/3)+(1+1/3)
= (4+8-11+1)+(0.4-0.1)+(-2/3+1/3)
=2+3/10-1/3
=1+(30+9-10)/30
=1+29/30
19,
=(1-2)-(3-4)+(5-6)-(7-8)+(9-10)-(11-12)+...+(2009-2010)
=-1-(-1)+(-1)-(-1)+(-1)-(-1)+...+(-1)
=[-1-(-1)]+[(-1)-(-1)]+[(-1)-(-1)]+...+[(-1)]
=-1
= (4+0.4)+(-0.1)+(8+1/3)-(11+2/3)+(1+1/3)
= (4+8-11+1)+(0.4-0.1)+(-2/3+1/3)
=2+3/10-1/3
=1+(30+9-10)/30
=1+29/30
19,
=(1-2)-(3-4)+(5-6)-(7-8)+(9-10)-(11-12)+...+(2009-2010)
=-1-(-1)+(-1)-(-1)+(-1)-(-1)+...+(-1)
=[-1-(-1)]+[(-1)-(-1)]+[(-1)-(-1)]+...+[(-1)]
=-1