[three]

ข้อความเดิมเขียนโดยคุณ banker

$(a + \dfrac{1}{b})(b+\dfrac{1}{c})(c+ \dfrac{1}{a}) = 5 \times 12 \times 13$

$abc + (a+b+c+ \dfrac{1}{a} +\dfrac{1}{b} +\dfrac{1}{c}) + \dfrac{1}{abc} = 780$

$abc+ (5+12+13) + \dfrac{1}{abc} = 780 $

$abc + \dfrac{1}{abc} = 780 - 30 = 720 $

ตอบ ข้อ 1.

780-30=750?