Matlab本身提供了丰富的数据类型（搜索matlab数据类型能找到详细的介绍），默认的数据类型是double型。
但是偶尔也会出现数据类型精度不够的情况，比如：计算斜率、倒数时经常出现“差之毫厘谬以千里”的现象。
因此需要提高现有数据类型的精度，matlab提供了可以自定义数据精度的函数vpa——详见Product Help或help。
需要说明的是：当不再需要高精度时，需要double(数据) -> double类型，或者其他你需要的类型。
>> help vpa
VPA Variable precision arithmetic.
R = VPA(S) numerically evaluates each element of the double matrix
S using variable precision floating point arithmetic with D decimal
digit accuracy, where D is the current setting of DIGITS.
The resulting R is a SYM.
VPA(S,D) uses D digits, instead of the current setting of DIGITS.
D is an integer or the SYM representation of a number.
It is important to avoid the evaluation of an expression using double
precision floating point arithmetic before it is passed to VPA.
For example,
phi = vpa((1+sqrt(5))/2)
first computes a 16-digit approximation to the golden ratio, then
converts that approximation to one with d digits, where d is the current
setting of DIGITS. To get full precision, use unevaluated string or
symbolic arguments,
phi = vpa('(1+sqrt(5))/2')
or
s = sym('sqrt(5)')
phi = vpa((1+s)/2);
Additional examples:
vpa(pi,780) shows six consecutive 9's near digit 770 in the
decimal expansion of pi.
vpa(hilb(2),5) returns
[ 1., .50000]
[.50000, .33333]
See also double, digits.
Overloaded methods:
sym/vpa
Reference page in Help browser
doc vpa
