a: a+a+1
b: a+a+1
c: 1/a+1/b
d: \(\left(2k+1+2k+3\right)^2\)
a. \(x+\left(x+1\right)\left(x\in N\right)\)
b. \(y+\left(y+1\right)\left(y\in Z\right)\)
c. \(\dfrac{1}{a}+\dfrac{1}{b}\left(a,b\in R;a,b\ne0\right)\)
d. \(\left(2n+1\right)^2+\left(2n+3\right)^2\left(n\in Z\right)\)