標題:
送10分!!!!!! 數學題(easy)奧maths7
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這些題目我都識答，不過想考考大家，送 吓分咁la!! (1) (1+3+5+7+7+5+3+1)/888888888= (2) 1/20+1/30+1/42+1/56+1/72= (3) 1/21+202/2121+50505/212121+13131313/21212121-1111/2121=
最佳解答:
(1) (1+3+5+7+7+5+3+1) / 888888888 = 32 / 888888888 = ( 8 x 4 ) / (8 x 111111111) = 4 / 111111111 (2) 1/20 + 1/30 + 1/42 + 1/56 +1/72 = 1 / (4 x 5 ) + 1 / (5 x 6) + 1 / (6 x 7) + 1 / (7 x 8) + 1 / (8 x 9) = 1 / 4 - 1 / 5 + 1 / 5 - 1 / 6 + 1 / 6 - 1 / 7 + 1 / 7 - 1 / 8 + 1 / 8 - 1 / 9 = 1 / 4 - 1 / 9 = 9 / 36 - 4 / 36 = 5 / 36 (3) 1/21+202/2121+50505/212121+13131313/21212121-1111/2121 = 1 / 21 + (101 x 2 ) / (101 x 21) + (10101 x 5) / (10101 x 21) + (1010101x 13) / (1010101 x 21) - (11 x 101)/ (101 x 21) = 1 / 21 + 2 / 21 + 5 / 21 + 13 / 21 - 11 / 21 = (1 + 2 + 5 +13 - 11) / 21 = 10 / 21
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