" If "1,m,n" are three lines such that "||m" and "n perp1" ,prove that "n perp m" ."

If l,n are three lines such that lm and n perp l prove that n perp m.

If l ,\ m,\n are three lines such that l || m\ a n d\ n_|_l , prove that n_|_mdot

If l,m and n are three straight lines such that l||m and l||n then prove that m||n .

If 1;m;n are lines in the same plane such that 1 intesects m and n|m ; then 1 intersects n also.

ABCD is a rectangle.Points M and N are on BD such that AM perp BD and CN perp BD Prove that BM^(2)+BN^(2)=DM^(2)+DN^(2)

If (l_1, m_1, n_1), (l_2, m_2, n_2)" and "(l_3, m_3, n_3) are the direction cosines of three mutually perpendicular lines, prove that the line whose direction cosines are proportional to (l_1+l_2+l_3,m_1+m_2+m_3,n_1+n_2+n_3) makes equal angles with them,

If the direction cosines of a straight line are l,m and n, then prove that l^(2)+m^(2)+n^(2)=1

If m and n are positive integers, then prove that the coefficients of x^(m) " and " x^(n) are equal in the expansion of (1+x)^(m+n)

If m and n are positive integers, then prove that the coefficients of x^(m) " and " x^(n) are equal in the expansion of (1+x)^(m+n)

If m parallel lines in a plane are intersected by a family of n parallel lines, the number of parallelograms that can be formed is a. 1/4m n(m-1)(n-1) b. 1/4m n(m-1)(n-1) c. 1/4m^2n^2 d. none of these