习题10-4 递归求简单交错幂级数的部分和

double fn(double x, int n) {
    double power_series_sumsum;
    if (n % 2 == 0 && n!=1) {
        power_series_sumsum = fn(x, n - 1) - pow(x,n);
    } else if (n % 2 == 1 && n != 1) {
        power_series_sumsum = fn(x, n - 1) + pow(x, n);
    } else if (n == 1) {
        power_series_sumsum = x;
    }
    return power_series_sumsum;
}
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