1 . tính nhanh:
a, A= 1/3x5 + 1/5x7 + .................+ 1/99x101
b, B = 5/1x2 + 5/2x3 + 5/3x4 + ................+ 99/100
c, C = 1/4x7 + 1/7x10 + 1/10x13 + ..................+ 1/40x43
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