(Solved): To develop a confidence interval for the difference between the means of two ...

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To develop a confidence interval for the difference between the means of two populations which are normally distributed and have equal variances, two independent samples of sizes $\mathrm{n}1$ and $\mathrm{n}2$ are randomly selected from the two populations. What is the formula used to determine the number of degrees of freedom for the $t$ distribution? a. $\mathrm{n}1+\mathrm{n}2$ b. $\mathrm{n}1+\mathrm{n}2?2$ c. $\mathrm{n}1+\mathrm{n}2+1$ d. $\mathrm{n}1+\mathrm{n}2?1$