Be a little more careful with your "minus" signs. If cos(v) = -24/25, then [cos(v)]^2 = [-24/25]^2, not -[24/25]^2. The square is onallof the value, not just the fraction!

It wasn't that the 1 "became" a cosine squared. It's that they converted the "1" to the common denominator:

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Hope that helps!