Find the vertex of this parabola
y=-x^2+8x-25

Is δ pmr similar to δ smn ? if so, which postulate or theorem proves these two triangles are similar?

Prove sin2a + cos2a - 1 / sin2a + cos2a + 1 = 1 - tana / 1 + cota

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! )