Đáp án:
Giải thích các bước giải:
`b, x + ( x + 1 ) + ( x + 2 ) +..........+ ( x + 30 ) = 1240`
`x . [( 30 - 0) : 1 + 1] + ( 1 + 2 + 3 +..........+ 30 ) = 1240`
`x . 31 + [( 30 - 1 ) : 1 + 1] . ( 30 + 1 ) : 2 = 1240`
`x . 31 + 30 . 31 : 2 = 1240`
`x . 31 + 465 = 1240`
`x . 31 = 1240 - 465`
`x . 31 = 775`
`x = 775 : 31`
`x = 25`
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`n, 1/2 + 1/12 + 1/24 + 1/40 + 1/60 + 1/84 + x = 1`
`1/2 . ( 1 + 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6 + 1/6.7 ) + x = 1`
`1/2 . ( 1 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 ) + x = 1`
`1/2 . ( 1 - 1/7 ) + x = 1`
`1/2 . 6/7 + x = 1`
`3/7 + x = 1`
`x = 1 - 3/7`
`x = 4/7`
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`m, 1 + 2 + 3 +............+ x = \overline{aaa}`
-Số số hạng là:
`( x - 1 ) : 1 + 1 = x` ( số hạng )
`→ ( x + 1 ) . x : 2 = 111a`
`→ ( x + 1 ) . x = 111a . 2`
`→ ( x + 1 ) . x = 222a`
`→ ( x + 1 ) . x = 6 . 37 . a`
`→ ( x + 1 ) . x = 36 . 37 . 6a`
`→ x = 36`
`→ ( 36 . 37 ) : 222 = 6`
`→ a = 6`
Vậy `a = 6; x = 36`
