Which of the following statement is correct
- $\triangle (U_k V_k) = U_k \triangle V_k + V_k \triangle U_k$
- $\triangle (U_k V_k) = U_{k+1} \triangle V_k + V_{k+1} \triangle U_k$
- $\triangle (U_k V_k) = V_{k+1} \triangle U_k + U_k \triangle V_k$
- $\triangle (U_k V_k) = U_{k+1} \triangle V_k + V_k \triangle U_k$