Non-zero numbers $a_1,a_2,a_3,\cdot \cdot \cdot a_n$ are in Harmonic Progression (H.P) if $\dfrac{1}{a_1},\dfrac{1}{a_2},\dfrac{1}{a_3},\cdot\cdot \cdot \dfrac{1}{a_n}$ are in A.P
If a, (a+d), (a+2d),... are in A.P,$\\$ $ n^{th} \text{ term of the A.P = a + (n - 1)d } \\$ Hence,if are in HP, $\dfrac{1}{a},\dfrac{1}{a+d},\dfrac{1}{a+2d},\cdot\cdot \cdot$ are in H.P,$\\ n^{th} \text{ term of the HP} = \dfrac{1}{a+(n-1)d}$
If a, b, c are in H.P,$~\dfrac{2}{b}=\dfrac{1}{a}+\dfrac{1}{c}$
The Harmonic Mean (HM) between two numbers a and b =$~\dfrac{2ab}{a+b}$