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一、递等式计算,能简便的用简便方法计算。
$2.4+13.8+7.6$
$3500÷1.25$
$6.8÷2.5×4$
$2+1.44÷(1.6×3)$
$2.4+13.8+7.6$
$3500÷1.25$
$6.8÷2.5×4$
$2+1.44÷(1.6×3)$
答案
$2.4 + 13.8 + 7.6$
$= (2.4 + 7.6) + 13.8$
$= 10 + 13.8$
$= 23.8$
$3500 ÷ 1.25$
$= (3500 × 8) ÷ (1.25 × 8)$
$= 28000 ÷ 10$
$= 2800$
$6.8 ÷ 2.5 × 4$
$= 2.72 × 4$
$= 10.88$
$2 + 1.44 ÷ (1.6 × 3)$
$= 2 + 1.44 ÷ 4.8$
$= 2 + 0.3$
$= 2.3$
$= (2.4 + 7.6) + 13.8$
$= 10 + 13.8$
$= 23.8$
$3500 ÷ 1.25$
$= (3500 × 8) ÷ (1.25 × 8)$
$= 28000 ÷ 10$
$= 2800$
$6.8 ÷ 2.5 × 4$
$= 2.72 × 4$
$= 10.88$
$2 + 1.44 ÷ (1.6 × 3)$
$= 2 + 1.44 ÷ 4.8$
$= 2 + 0.3$
$= 2.3$
二、列综合算式计算或列方程求解。
1. 甲数是80,乙数比甲数的3倍多10,乙数是多少?
1. 甲数是80,乙数比甲数的3倍多10,乙数是多少?
答案
80×3 + 10
=240 + 10
=250
答：乙数是250。
=240 + 10
=250
答：乙数是250。
2. 一个数的4倍减去1.5的差正好等于这个数,求这个数。
答案
解：设这个数为x。
4x - 1.5 = x
4x - x = 1.5
3x = 1.5
x = 0.5
答：这个数是0.5。
4x - 1.5 = x
4x - x = 1.5
3x = 1.5
x = 0.5
答：这个数是0.5。
三、应用题。
一个梯形的上底是8厘米，如果把下底延长3厘米就会成为一个平行四边形，面积会比原来增加24平方厘米，求原来梯形的面积。
一个梯形的上底是8厘米，如果把下底延长3厘米就会成为一个平行四边形，面积会比原来增加24平方厘米，求原来梯形的面积。
答案
24×2÷3=16（厘米）
8-3=5（厘米）
(8+5)×16÷2=104（平方厘米）
答：原来梯形的面积是104平方厘米。
8-3=5（厘米）
(8+5)×16÷2=104（平方厘米）
答：原来梯形的面积是104平方厘米。
第16页“动脑筋”的答案：$(n-1)+n+(n+1)=3n-1+1=3n$
因为$3n$是3的倍数，所以三个连续自然数相加的和一定是3的倍数。
选择填空。Choose the best answer.
()(1) You can't eat drink at the Art Museum.
A. and B. or C. but
()(2) Father and mother both in the bedroom.
A. were B. was C. is
()(3) His bag old, but now he a new one.
A. is; has B. was; had C. was; has
()(4) Our teachers are waiting us at the gate.
A. for B. with C. to
()(5) Here was the weather Sunday.
A. on B. for C. to
()(6) Would you like tea for breakfast?
A. many B. little C. some
()(7) The giant the wall down last week.
A. broke B. broken C. breaks
()(8) The robots in the museum are very .
A. use B. usefully C. useful
()(9) At Halloween, there are jack-o'-lanterns .
A. anywhere B. everywhere C. whatever
()(10) We like having “” parties with friends at home.
A. fancy-dress B. fancy-dressing C. fancy-dressed
()(11) Yesterday we go to school.
A. aren't B. didn't C. don't
()(12) It was a tadpole. Now it is a .
A. butterfly B. frog C. caterpillar
()(13) A typhoon is coming from the China Sea.
A. east B. / C. East
()(14) Many years ago, not many people in Shanghai.
A. live B. lived C. lives
()(15) The piano prince is playing the piano. He is cool.
A. really B. real C. ready
因为$3n$是3的倍数，所以三个连续自然数相加的和一定是3的倍数。
选择填空。Choose the best answer.
()(1) You can't eat drink at the Art Museum.
A. and B. or C. but
()(2) Father and mother both in the bedroom.
A. were B. was C. is
()(3) His bag old, but now he a new one.
A. is; has B. was; had C. was; has
()(4) Our teachers are waiting us at the gate.
A. for B. with C. to
()(5) Here was the weather Sunday.
A. on B. for C. to
()(6) Would you like tea for breakfast?
A. many B. little C. some
()(7) The giant the wall down last week.
A. broke B. broken C. breaks
()(8) The robots in the museum are very .
A. use B. usefully C. useful
()(9) At Halloween, there are jack-o'-lanterns .
A. anywhere B. everywhere C. whatever
()(10) We like having “” parties with friends at home.
A. fancy-dress B. fancy-dressing C. fancy-dressed
()(11) Yesterday we go to school.
A. aren't B. didn't C. don't
()(12) It was a tadpole. Now it is a .
A. butterfly B. frog C. caterpillar
()(13) A typhoon is coming from the China Sea.
A. east B. / C. East
()(14) Many years ago, not many people in Shanghai.
A. live B. lived C. lives
()(15) The piano prince is playing the piano. He is cool.
A. really B. real C. ready
答案
(1) B
(2) A
(3) C
(4) A
(5) A
(6) C
(7) A
(8) C
(9) B
(10) A
(11) B
(12) B
(13) C
(14) B
(15) A
(2) A
(3) C
(4) A
(5) A
(6) C
(7) A
(8) C
(9) B
(10) A
(11) B
(12) B
(13) C
(14) B
(15) A
