1.

If a and b are two vectos such that a.b lt 0 and |a.b|=|axxb|, then the angle between a and b is

Answer»

`(3pi)/(4)`
`(2pi)/(3)`
`(pi)/(4)`
`(pi)/(3)`

Solution :Given,`|a *b| = | axxb|`
` RARR |a||b||cos THETA|=|a||b||SIN theta|`
[ where,`theta`is the angle between a and b]
`rArr |cos theta| = |sin theta| `
`rArrtheta = (pi)/(4) or (3pi)/(4) "" [ "as " 0 le theta le pi ]`
But`a*b lt 0 `
`therefore theta=(3pi)/(4) `


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