杨辉三角系数表（a+b）的5次方展开

（A+B)^5
=(A+B)^2(A+B)^2(A+B)
=(A^2+2AB+B^2)(A^2+2AB+B^2)(A+B)
=(A^4+4A^3*B+6A^2*B^2+4AB^3+B^4)(A+B)=A^5+5A^4B+10A^3B^2+10A^2B^3+5AB^4+B^5