计算数学表达式，支持常用函数、常数与单位换算
2 + 3 * 4
sqrt(16) + 2^3
sin(pi/2) + cos(0)
log(1000, 10) + sqrt(9)
abs(-5) + round(3.7)
12 inch to cm
5 km + 2 mile to m
factorial(5)
gcd(12, 8) + lcm(4, 6)
sqrt
pow / ^
abs
round
floor
ceil
sin/cos/tan
log
log / ln
exp
factorial
pi
e
phi