(1) If an equation (i:e f(x)=0 ) contains all positive co-efficients of any powers of x , it has no positive roots then.

eg: x^4+3x^2+2x+6=0 has no positive roots .

(2) For an equation , if all the even powers of x have some sign coefficients and all the odd powers of x have the opposite sign coefficients , then it has no negative roots .

(3)Summarising DESCARTES RULE OF SIGNS:

For an equation f(x)=0 , the maximum number of positive roots it can have is the number of sign changes in f(x) ; and the maximum number of negative roots it can have is the number of sign changes in f(-x) .

Hence the remaining are the minimum number of imaginary roots of the equation(Since we also know that the index of the maximum power of x is the number of roots of an equation.)

(4) Complex roots occur in pairs, hence if one of the roots of an equation is 2+3i , another has to be 2-3i and if there are three possible roots of the equation , we can conclude that the last root is real . This real roots could be found out by finding the sum of the roots of the equation and subtracting (2+3i)+(2-3i)=4 from that sum. (More about finding sum and products of roots next time )

(5) For a cubic equation ax^3+bx^2+cx+d=o

sum of the roots = – b/a

sum of the product of the roots taken two at a time = c/a

product of the roots = -d/a

(6) For a biquadratic equation ax^4+bx^3+cx^2+dx+e = 0

sum of the roots = – b/a

sum of the product of the roots taken three at a time = c/a

sum of the product of the roots taken two at a time = -d/a

product of the roots = e/a

(7) If an equation f(x)= 0 has only odd powers of x and all these have the same sign coefficients or if f(x) = 0 has only odd powers of x and all these have the same sign

coefficients then the equation has no real roots in each case(except for x=0 in the second case.

(8) Besides Complex roots , even irrational roots occur in pairs. Hence if 2+root(3) is a root , then even 2-root(3) is a root .

(All these are very useful in finding number of positive , negative , real ,complex etc roots of an equation )

(9) Consider the two equations

a1x+b1y=c1

a2x+b2y=c2

Then ,

If a1/a2 = b1/b2 = c1/c2 , then we have infinite solutions for these equations.

If a1/a2 = b1/b2 <> c1/c2 , then we have no solution for these equations.(<> means not equal to )

If a1/a2 <> b1/b2 , then we have a unique solutions for these equations..