(1) x = 1,y = 1 (与a的值无关,A(1,1)
(2) 带入直线方程:m + n- 1 = 0
n = 1 - m
1/m + 1/n = (m + n)/(mn) = 1/(mn)
1/m+1/n取最小值时,mn取最大值
mn = m(1 - m) = -(m - 1/2)² + 1/4
mn最大值为1/4,1/m+1/n最小值为 1/(1/4) = 4