1/12+1/20 +1/30+1/42 + ... + 1/182 = 1/(3*4) + 1/(4*5) +1/(6*7) +... +1/(13*14) =
=(1/3 - 1/4) + (1/4 - 1/5) +(1/6 -1/7) + ...+(1/13 - 1/14) =1/3 - 1/14 = 11/42 .
18,36+0,64:0,8=18,36+0,8=19,16
7/15< а/15 <1
умножим на 15
7< а <15
Ответ. 7< а <15
X^2 - x -2 = 0
x^2 -x = 2 /:x
x - 1 = 2/x
x = 2/x +1
x = 2