Question Number 61948 by subhankar10 last updated on 12/Jun/19
$$\mathrm{The}\:\mathrm{vectors}\:\boldsymbol{\mathrm{a}},\:\boldsymbol{\mathrm{b}},\:\boldsymbol{\mathrm{c}}\:\mathrm{are}\:\mathrm{equal}\:\mathrm{in}\:\mathrm{length} \\ $$$$\mathrm{and}\:\mathrm{taken}\:\mathrm{pairwise},\:\mathrm{they}\:\mathrm{make}\:\mathrm{equal} \\ $$$$\mathrm{angles}.\:\mathrm{If}\:\:\boldsymbol{\mathrm{a}}=\boldsymbol{\mathrm{i}}+\boldsymbol{\mathrm{j}},\:\:\boldsymbol{\mathrm{b}}=\boldsymbol{\mathrm{j}}+\boldsymbol{\mathrm{k}}\:\:\mathrm{and}\:\:\boldsymbol{\mathrm{c}}\:\mathrm{makes} \\ $$$$\mathrm{an}\:\mathrm{obtuse}\:\mathrm{angle}\:\mathrm{with}\:{X}−\mathrm{axis},\:\mathrm{then}\:\boldsymbol{\mathrm{c}}= \\ $$