5x+cosx=0show that the equation 5x+cosx=0 has a solution.Are

令f（x）=5x+cosx
f'（x）=5-sinx,由于·-1≤sinx≤1
所以5-sinx＞0
于是f（x）为单调增函数
于是f（x）=0仅有一解
即5x+cosx=0仅有一根