Suppose a certain type of fertilizer has an expected yield per acre of , with variance o, whereas the expected yield for a second type of fertilizer is , with the same variance . Let S and S denote the sample variances of yields based on sample sizes n and n, respectively, of the two fertilizers. Show that the pooled (combined) estimator 82 (n-1)+(n - 1)S is an unbiased estimator of o.