Toán Lớp 5: tinh nhanh 2/3 +2/6+2/12+2/24+2/48+2/96+2/192
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Toán Lớp 5: tinh nhanh 2/3 +2/6+2/12+2/24+2/48+2/96+2/192
Comments ( 2 )
= 2/3 xx (1 + 1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64)
Xét phần trong ngoặc: (1 +1/2 +1/4+1/8+1/16+1/32+1/64)
1/2 + 1/4 = 1 – 1/4
1/2 + 1/4 + 1/8 = 1 – 1/8
1/2 + 1/4 + 1/8 + 1/16 + 1/32 + 1/64 = 1 – 1/64 = 63/64
Suy ra 1 + 63/64 = 127/64
2/3 + 2/6 + 2/12 + 2/24 + 2/48 + 2/96 + 2/192
= 2/3 xx 127/64 = 127/96