If 1/x + 1/y = 3 and 1/x - 1/y = 7, what is the value of x + y? Express your answer as a common fraction.
1/x + 1/y = 3
1/x - 1/y = 7 add these
2 / x = 10
x = 2/10 = 1/5
And
1 / (1/5) + 1/y = 3
5 + 1/y= 3
1/y = 3 -5
1/y = -2
y = -1/2
x + y = 1/5 - 1/ 2 = -3 / 10