Toán Lớp 8: (x – 1) + (x – 2) + (x – 3) + ……….+ (x – 10) = 15 × 25 × 4 + 75
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Toán Lớp 8: (x – 1) + (x – 2) + (x – 3) + ……….+ (x – 10) = 15 × 25 × 4 + 75
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$\text{(x – 1) + (x – 2) + (x – 3) + ……….+ (x – 10) = 15 × 25 × 4 + 75 }$
$\text{⇔ x + x + …+ x – 1 – 2 – 3 – 4 …. -10 }$
$\text{= 15 . 100 + 75 }$
$\text{⇔ 10 x + ( -1 – 9 ) + ( – 2 – 8 ) + ( – 3 – 7 ) + ( – 4 – 6 ) + ( – 5 – 10 ) }$
$\text{= 1500 + 75 }$
$\text{⇔ 10 x – 10 . 4 – 15 = 1575}$
$\text{⇔ 10x – 55 = 1575}$
$\text{⇔ 10x = 1575 + 55}$
$\text{⇔ 10x = 1630 }$
$\text{⇔ x = 1630 : 10}$
$\text{suy ra x = 163 }$