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Continuity And Differentiability

Question
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If y = a sin x + b cos x, prove that straight y squared plus open parentheses dy over dx close parentheses squared equals straight a squared plus straight b squared.

Solution
Here space space straight y equals straight a space sin space straight x plus straight b space cos space straight x space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space... left parenthesis 1 right parenthesis
therefore space dy over dx equals straight a space cos space straight x minus straight b space sin space straight x space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space space... left parenthesis 2 right parenthesis
Squaring space and space adding space left parenthesis 1 right parenthesis space and space left parenthesis 2 right parenthesis comma space we space get comma
space space space space space space space space space straight y squared plus open parentheses dy over dx close parentheses squared equals straight a squared left parenthesis sin squared straight x plus cos squared space straight x right parenthesis plus straight b squared left parenthesis cos squared straight x plus sin squared straight x right parenthesis
therefore space space space space space straight y squared plus open parentheses dy over dx close parentheses squared equals straight a squared left parenthesis 1 right parenthesis plus straight b squared left parenthesis 1 right parenthesis
therefore space space space space space straight y squared plus open parentheses dy over dx close parentheses squared equals straight a squared plus straight b squared

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