Toán Lớp 6: a) 1×3+3×5+5×7+7×9+…+99×101
b) 1×2+2×3+3×4+99×100
giúp với nha, và giải thích nữa nha
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Toán Lớp 6: a) 1×3+3×5+5×7+7×9+…+99×101
b) 1×2+2×3+3×4+99×100
giúp với nha, và giải thích nữa nha
Comments ( 2 )
A=1xx3 +3xx5 +5xx7 +….+99xx101
6A=1xx3xx(5+1) + 3xx5xx(7-1) +5xx7xx(9-3) +…+ 99xx101xx(103-97)
6A=3+ 1xx3xx5 +3xx5xx7-1xx3xx5 + 5xx7xx9 -3xx5xx7 +….+99xx101xx103 – 97xx99xx101
6A=3+99xx101xx103=1019703
=>A=1019703/6
b) Đặt A
A = 1xx2 + 2xx3 + 3xx4 + 4xx5 + …+ 99xx100
A3 = 1xx2xx3 + 2xx3xx3+ 3xx4xx3 + 4xx5xx3 + … + 99xx100xx3
A3 = 1xx2xx3 + 2xx3xx(4-1) + 3xx4xx(5-2) + 4xx5xx(6-3) + … + 99xx100xx(101-98)
A3 = 1xx2xx3 + 2xx3xx4 – 1xx2xx3 + 3xx4xx5 – 2xx3xx4+ 4xx5xx6 – 3xx4xx5 + … + 99xx100xx101 – 98xx99xx100
A3 = 99xx100xx101
A= (99xx100xx101)/3
A = 999900/3
=>A=333300