Question

Ex 12,13 cat mai repede va rog!

Answer 1

Răspuns:

Explicație pas cu pas:

12.

b1 = 2

b2 = 6 = b1*q

6 = 2*q

q = 6 : 2 = 3

b3 = b2*q = 6*3 = 18

b4 = b3*q = 18*3 = 54

13.

r = 3

S10 = 145

Sn = n*(a1 + an)/2

an = a1 + (n - 1)r

a10 = a1 + 9r = a1 + 9*3 = a1 + 27

S10 = 10*(a1 + a10)/2

145 = 10*(a1 + a1 + 27)/2

290 = 10*(2a1 + 27)

29 = 2a1 + 27

2a1 = 29 - 27 = 2

a1 = 2 : 2 = 1

Answer 2

$\it 12.\\ \\ q=b_2:b_1=6:2=3\\ \\ b_4=b_2\cdot q^2=6\cdot3^2=54$

$\it 13.\\ \\ S_{10}=\dfrac{(a_1+a_{10})\cdot10}{2} \Rightarrow 145=(a_1+a_{10})\cdot5\bigg|_{:5} \Rightarrow a_1+a_{10}=29 \Rightarrow \\ \\ \\ \Rightarrow a_1+a_1+9r=29\Rightarrow 2a_1+9\cdot3=29\Rightarrow \bigg|_{-27}\Rightarrow 2a_1=2\Rightarrow a_1=1$