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

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

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答案因f(t)连续 [*]f(s)sinsds可导[*]f(t)可导.于是,将题设等式两边求导可得 f′(t)=-2sin2t+f(t)sint,即f′(t)-f(t)sint=-2sin2t,又f(0)=1. 这是一阶线性微分方程的初值问题.将方程两边乘μ=e-∫sintdt=ecost可得 [ecostf(t)]′=-4sintcostecost. 积分得 ecostf(t)=4∫costd(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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