设f(t)连续并满足f(t)=cos2t+∫0tf(s)sinsds,求f(t).

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问题 设f(t)连续并满足f(t)=cos2t+∫0tf(s)sinsds,求f(t).

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答案因f(t)连续,故∫0tf(s)sinsds可导,从而f(t)可导.于是,将题设等式两边求导并在题设等式中令t=0,可得 [*] 这是一阶线性微分方程的初值问题.将方程两边同乘μ=e—∫sintdt=ecost可得 [ecostf(t)]’=一4sintcostecost. 积分得 ecostf(t)=4∫costd(ecost)=4(cost一1)ecost+C. 由f(0)=1得C=e.因此所求函数f(t)=e1—cost+4(cost一1).

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