Can somebody explain how the derivative of (cost-sint)i and (cost+sint)j equal -2sinti and 2costj. Should it be (-sint-cost)i and(-sint-ccost)j?
Close. It should be (-sint-cost)i and(-sint+cost)j
No idea how they came up with their answers.
I will note that cost-sint = √2 sin(π/4 - t) but that may be of no help.