Answer:
These are all quadratics, where the x/y/a/b coefficient has been cancelled out. So it's like factorising a quadratic. What i mean by the x/y/a/b coefficient being cancelled out is for example y^2 - 1, becomes (y - 1)(y + 1). This is equal to y^2 - y + y - 1. You can see that - y + y = 0, to therefore it's not visible in the final equation, but it's there.
c) y^2 - 1 = (y - 1)(y + 1)
d) x^2 - 64 = (x - 8)(x + 8)
g) a^2 - 81 = (a - 9)(a + 9)
h) x^2 - y^2 = (x - y)(x + y)
k) 25 - x^2 = (5 - x)(5 + x)
l) 1 - b^2 = (1 - b)(1 + b)
o) x^2 - 400 = (x - 20)(x + 20)
p) 900 - y^2 = (30 - y)(30 + y)
I hope this makes sense :))
