(bad) intuition: modulates the frequency term in the sin.
6
Expression 7: sine left parenthesis, left parenthesis, 10 plus "a" sin left parenthesis, "x" , right parenthesis , right parenthesis times "x" , right parenthesis minus 2.5sin10+asinx·x−2.5
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Correct solution: find the current phase through integration, then apply the sin.
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Expression 9: phi left parenthesis, "x" , right parenthesis equals 10 "x" plus "a" cosine left parenthesis, "x" , right parenthesisϕx=10x+acosx
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Expression 10: sine left parenthesis, phi left parenthesis, "x" , right parenthesis , right parenthesis minus 5sinϕx−5