Explain why (a) ???? = ???????? ????????+???????? ????????/????????+???????? is used in computing the standard error of ????????

Explain why

(a) ˆ???? = ???????? ˆ????????+???????? ˆ????????/????????+????????

is used in computing the standard error of ˆ???????? − ˆ???????? in the large sample ????-test statistic for a hypothesis test about the difference between two population proportions?

(b) ˆ???????? and ˆ???????? are used in computing the standard error of ˆ????????−ˆ???????? in the large sample confidence interval for the difference between two population proportions?