Question

4+7+10+...x=144. помогите пожалуйста. заранее спасибо.

Answer 1

a1 = 4
a2 = 7
d =a2 - a1 = 7 - 4 = 3
Sn = 144
an - ?

Sn = (a1 + an)/2*n = 144
(a1 + a1 +d(n - 1))/2* n = 144
(2a1 + d(n - 1))/2* n = 144
(8 + 3(n - 1))/2 * n = 144
(3n + 5)/2* n = 144
(3n + 5)*n/2  - 144 = 0 
(3n + 5)*n  - 288 = 0 
3n^2 + 5n - 288 = 0 
D = 3481 = 59^2
n1 = ( - 5 + 59)/6 = 54/6 = 9
n2 = ( - 5 - 59)/6 <  0

a9 = a1 + 8d = 4 + 8*3 = 28