a)Ta có :\(\widehat{C2}\)=\(\widehat{C4}\)=110⁰(2 góc đối đỉnh)

Vì đường thẳng d cắt a, b và tạo ra 1 cặp góc trong cùng phía bù nhau (\(\widehat{C4}+\widehat{D1}=180^0\))\(\Rightarrow\)a//b

b)Vì a\(\perp\)c ;a//b \(\Rightarrow\)c\(\perp\)b

Qua 1 điểm nằm ngoài 1 đường thẳng ,chỉ có 1đường thẳng song song với đường thẳng đã cho.

Ta có:\(\dfrac{1}{1^2}\)<\(\dfrac{1}{1\times2}\);\(\dfrac{1}{2^2}\)<\(\dfrac{1}{2\times3}\);...................;\(\dfrac{1}{2017^2}\)<\(\dfrac{1}{2017\times2018}\)

\(\Rightarrow\)\(\dfrac{1}{1^2}\)\(+\)\(\dfrac{1}{2^2}\)\(+\)..........\(+\)\(\dfrac{1}{2017^2}\)<\(\dfrac{1}{1\times2}\)\(+\)\(\dfrac{1}{2\times3}\)\(+\).........\(+\)\(\dfrac{1}{2017\times2018}\)

\(\Rightarrow\)\(\dfrac{1}{1^2}\)\(+\)\(\dfrac{1}{2^2}\)\(+\)..........\(+\)\(\dfrac{1}{2017^2}\)<\(\dfrac{1}{1}\)\(-\)\(\dfrac{1}{2}\)\(+\)\(\dfrac{1}{2}\)\(-\)\(\dfrac{1}{3}\)\(+\).........\(+\)\(\dfrac{1}{2017}\)\(-\)\(\dfrac{1}{2018}\)

\(\Rightarrow\)\(\dfrac{1}{1^2}\)\(+\)\(\dfrac{1}{2^2}\)\(+\)..........\(+\)\(\dfrac{1}{2017^2}\)<\(\dfrac{1}{1}\)\(-\)\(\dfrac{1}{2018}\)<2

\(\Rightarrow\)\(\dfrac{1}{1^2}\)\(+\)\(\dfrac{1}{2^2}+......+\dfrac{1}{2017^2}\)<2