https://www.wolframalpha.com/input?i=a%3D4%2Cb%3D5%2Cx%3Dacos%28%284%5E2%2B5%5E2-6%5E2%29%2F%282*4*5%29%29%E3%81%AE%E3%81%A8%E3%81%8D%E3%81%AEsqrt%28a%5E2%2Bb%5E2-2*a*b*cos%28x%2Bpi%2F3%29%29&lang=ja