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[*]∫0tf(s)sinsds可导[*]f(t)可导.于是 [*] 这是一阶线性微分方程的初值问题.方程两边乘μ=e-∫sintdt=eccost,得 [ecostf(t)]’=-4sintcostecost. 积分得ecostf(t)=4fcostd(ecost)=4(cost-1)ecost+C. 由f(0)=1得C=e.因此,f(t)=e1-cost+4(cost-1).

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