Which, ø, forms the point on the unit circle with the coordinates ( √2 / 2 , √2 /2) ?

I'm at a bookstore with $\$35$ in my pocket and a $20\%$-off coupon. what is the cost of the most expensive book i can buy with my money and the coupon? (assume that there are books of every possible price in the bookstore. it's a large bookstore! )