[tex]barisan \: dan \: deret \: [/tex] No 39 dan 40

39.) Misalkan
x2 = p
x3 = q
x4 = r
dgn persamaan
[tex](1)... \frac{p}{10} = \frac{q}{p} = \frac{r}{q} \\ (2)...(q - 10) - (p - 10) = (r - q - p - 10) - (q - 10) \\ 3q = r \\ \\ substitusi \: pers \: 2 \: ke \: pers \: 1 \\ \frac{p}{10} = \frac{3q}{q} \\ p = 30 \\ \\ maka \\ q = \frac{p {}^{2} }{10} = \frac{30 {}^{2} }{10} = 90 \\ r = \frac{pq}{10} = \frac{30 \times 90}{10} = 270 \\ \\ jadi \: \\ x4 = 270 [/tex]

40.)
4-a = b-4
4+4 = a+b
a + b = 8
[tex] \frac{3}{a} = \frac{b}{3} \\ 9 = ab \\ \\ maka \\ \frac{1}{a} + \frac{1}{b} = \frac{b}{ab} + \frac{a}{ab} \\ = \frac{a + b}{a \times b} \\ = \frac{8}{9} [/tex]